British-English Dictionary for UK Spell checking
https://addons.mozilla.org/en-GB/firefox/addon/british-english-dictionary-2/

I Don't care about cookies - Hide cookie confirmations on websites
https://addons.mozilla.org/en-GB/firefox/addon/i-dont-care-about-cookies/

uBlock Origin - Popup and Ad blocker
https://addons.mozilla.org/en-GB/firefox/addon/ublock-origin/

Measure IT - Ruler addon to measure width/height of areas on websites
https://addons.mozilla.org/en-GB/firefox/addon/measure-it/

Web Developer - adds various web developer tools to the browser.
https://addons.mozilla.org/en-GB/firefox/addon/web-developer/

ColorZilla - Advanced Eyedropper, Color Picker, Gradient Generator…
https://addons.mozilla.org/en-GB/firefox/addon/colorzilla/

Fireshot Lite - Full webpage screenshots or saving as PDF
https://addons.mozilla.org/en-GB/firefox/addon/fireshot/

HTML Validator - Validates HTML of websites
https://addons.mozilla.org/en-GB/firefox/addon/html-validator/

Vertical Tabs Reloaded
https://addons.mozilla.org/en-GB/firefox/addon/vertical-tabs-reloaded/

Mail Merge
https://addons.thunderbird.net/en-GB/thunderbird/addon/mail-merge/

Manually sort folders
https://addons.thunderbird.net/en-GB/thunderbird/addon/manually-sort-folders/

Lightning

Edit email subject
https://addons.thunderbird.net/en-GB/thunderbird/addon/edit-email-subject/reviews/

Mail Redirect
https://addons.thunderbird.net/en-GB/thunderbird/addon/mailredirect/
https://mailredirect.sourceforge.io/

Provider for Google Calendar
https://addons.mozilla.org/thunderbird/addon/4631

Open Google Calendar
https://addons.thunderbird.net/en-GB/thunderbird/addon/open-google-calendar/

LookOut (fix version)
https://addons.thunderbird.net/en-GB/thunderbird/addon/lookout-fix-version/
https://github.com/TB-throwback/LookOut-fix-version/

1. Via addon manager in TB, install tbsync addon as well as provider for exchange activesync.
   https://addons.thunderbird.net/en-GB/thunderbird/addon/eas-4-tbsync/
   https://addons.thunderbird.net/en-GB/thunderbird/addon/tbsync/
2. then go to tools → addon preferences → tbsync
3. account actions → add new account → exchange activesync
4. office365 account, give name and add work email
5. authenticate as work account with MS password
6. select the new account, enable sync, tick boxes of contacts and calendars, set periodic sync to 10 and click synchronize now button.
7. calendar entries now show up in lightning

not working as of Feb 2020:

• sending or responding to meeting invitations
• push sync
• Lightning sync buttons do not trigger a sync of EAS calendars

also, this only shows your own calendar, not any others that may have been added to outlook.

To have better visibility of labels of mails in the mail list, the style can be changed by editing or creating a chrome/userChrome.css file in the Thunderbird profile directory, then restarting Thunderbird (v68+).

~/.thunderbird/something.default/chrome/userChrome.css

/*
 * Label colors.  Highlight background rather than change font color.
 */
 
/* Label1 Important */
treechildren::-moz-tree-row(T_24label1) { background-color:  #FFCCCC !important; }
treechildren::-moz-tree-cell-text(T_24label1) { color: #000 !important; }
treechildren::-moz-tree-row(T_24label1, selected) { background-color: #DD0000 !important; }
treechildren::-moz-tree-cell-text(T_24label1, selected) { color: #FFF !important; }
 
/* Label2 Work */
treechildren::-moz-tree-row(T_24label2) { background-color:  #FFFFCC !important; }
treechildren::-moz-tree-cell-text(T_24label2) { color: #000 !important; }
treechildren::-moz-tree-row(T_24label2, selected) { background-color: #DDDD00 !important; }
treechildren::-moz-tree-cell-text(T_24label2, selected) { color: #FFF !important; }
 
/* Label3 Personal */
treechildren::-moz-tree-row(T_24label3) { background-color:  #99FF99 !important; }
treechildren::-moz-tree-cell-text(T_24label3) { color: #000 !important; }
treechildren::-moz-tree-row(T_24label3, selected) { background-color: #00CC00 !important; }
treechildren::-moz-tree-cell-text(T_24label3, selected) { color: #FFF !important; }
 
/* Label4 To Do */
treechildren::-moz-tree-row(T_24label4) { background-color:  #CCCCFF !important; }
treechildren::-moz-tree-cell-text(T_24label4) { color: #000 !important; }
treechildren::-moz-tree-row(T_24label4, selected) { background-color: #0000DD !important; }
treechildren::-moz-tree-cell-text(T_24label4, selected) { color: #FFF !important; }
 
/* Label5 Later */
treechildren::-moz-tree-row(T_24label5) { background-color:  #FFCCFF !important; }
treechildren::-moz-tree-cell-text(T_24label5) { color: #000 !important; }
treechildren::-moz-tree-row(T_24label5, selected) { background-color: #CC33CC !important; }
treechildren::-moz-tree-cell-text(T_24label5, selected) { color: #FFF !important; }
 
 
 
/*
 * Add vertical scrollbar for expanded header view
 */
/*
#msgHeaderView { 
        max-height: 15em !important;
        overflow: auto !important; 
}
*/
 
/*
 * Limit height of attachment window
 */
#attachmentView {
        -moz-appearance: none !important;
        height: 74px !important;
        overflow: auto !important;
}
 
/*
 * Highlight the drop folder when drag&drop mails to a folder 
 * (Proposed patch as per Bug 668631)
 * https://bugzilla.mozilla.org/show_bug.cgi?id=668631
 */
#folderTree treechildren::-moz-tree-cell(dropOn) {
    background-image: -moz-linear-gradient(rgba(255, 255, 255, .7),
                      rgba(255, 255, 255, .45));
    background-color: Highlight;
    border-radius: 3px;
}
 
/*
 * Replace priority text with icon as per 
 * http://kb.mozillazine.org/UserChrome.js/Mail
 */
#priorityCol, treechildren::-moz-tree-image(priorityCol)
 {list-style-image: url("Priority-icons-tb.png");}
 
treechildren::-moz-tree-image(priorityCol)
 {-moz-image-region: rect(0px 16px 16px 0px);}
treechildren::-moz-tree-image(priorityCol, priorityNone)
 {-moz-image-region: rect(0px 32px 16px 16px);}
treechildren::-moz-tree-image(priorityCol, priorityLowest)
 {-moz-image-region: rect(0px 48px 16px 32px);}
treechildren::-moz-tree-image(priorityCol, priorityLow)
 {-moz-image-region: rect(0px 64px 16px 48px);}
treechildren::-moz-tree-image(priorityCol, priorityNormal)
 {-moz-image-region: rect(0px 80px 16px 64px);}
treechildren::-moz-tree-image(priorityCol, priorityHigh)
 {-moz-image-region: rect(0px 96px 16px 80px);}
treechildren::-moz-tree-image(priorityCol, priorityHighest)
 {-moz-image-region: rect(0px 112px 16px 96px);}
 
#priorityCol
 {-moz-image-region: rect(0px 128px 16px 112px);
  -moz-binding: url("chrome://global/content/bindings/tree.xml#treecol-image");
  padding-left: 0 !important;
  width: 23px;
  max-width: 23px;}

Thunderbird 115+

~/.thunderbird/something.default/chrome/userChrome.css

/*
 * Label colors.  Highlight background rather than change font color.
 */
 
/* Label1 */
tr[data-properties~="T_24label1"] { background-color: #FFCCCC !important; color: #000 !important;}
tr[data-properties~="T_24label1"][aria-selected~="true"] { background-color: #DD0000 !important; color: #FFF !important;} 
 
/* Label2 */
tr[data-properties~="T_24label2"] { background-color: #FFFFCC !important; color: #000 !important; }
tr[data-properties~="T_24label2"][aria-selected~="true"] { background-color: #DDDD00 !important; color: #FFF !important; }
 
/* Label3 */
tr[data-properties~="T_24label3"] { background-color: #99FF99 !important; color: #000 !important; }
tr[data-properties~="T_24label3"][aria-selected~="true"] { background-color: #00CC00 !important; color: #FFF !important; }
 
/* Label4 */
tr[data-properties~="T_24label4"] { background-color: #CCCCFF !important; color: #000 !important; }
tr[data-properties~="T_24label4"][aria-selected~="true"] { background-color: #0000DD !important; color: #FFF !important; }
 
/* Label5 */
tr[data-properties~="T_24label5"] { background-color: #FFCCFF !important; color: #000 !important; }
tr[data-properties~="T_24label5"][aria-selected~="true"] { background-color: #CC33CC !important; color: #FFF !important; }

Thunderbird does not remember the last window and display position. Using wmctrl and xdotool the window position can be set and it can be snapped to use top half of left-most monitor. Adjust as necessary. The adjustment to thunderbird.sh probably will be overwritten on Thunderbird update. Alternative using wrapper shell script would need to be used if that's the case:

#General command line commands for testing:
#Set thunderbird window to top left most screen
wmctrl -i -r $(wmctrl -l | grep Thunderbird | cut -d " " -f1) -e 0,0,56,1200,1087
#Snap thunderbird window to top half of screen (keycombination windows+up)
xdotool windowactivate $(xdotool search --name thunderbird | tail -1) key "Super_L+Up"

#main Thunderbird.sh script adjustment:
sudo vi /usr/lib/thunderbird/thunderbird.sh 
add just before exec $MOZ_LIBDIR/$EXE "$@" at end of file:
    #Set thunderbird window to top left most screen
    (sleep 3;wmctrl -i -r $(wmctrl -l | grep Thunderbird | cut -d " " -f1) -e 0,0,56,1200,1087) &
    #Snap thunderbird window to top half of screen (keycombination windows+up)
    (sleep 3;xdotool windowactivate $(xdotool search --name thunderbird | tail -1) key "Super_L+Up") &

Systray-X is an app and Thunderbird plugin to add a systray icon and enable minimise to systray for TB.

https://github.com/Ximi1970/systray-x

Installation:

curl -sL -o- https://download.opensuse.org/repositories/home:/Ximi1970/xUbuntu_20.04/Release.key | gpg --dearmor | sudo tee /usr/share/keyrings/systray-x-keyring.gpg
echo "deb [signed-by=/usr/share/keyrings/systray-x-keyring.gpg] https://download.opensuse.org/repositories/home:/Ximi1970:/Mozilla:/Add-ons/xUbuntu_20.04 ./" | sudo tee /etc/apt/sources.list.d/systray-x.list
sudo apt update

sudo apt install systray-x-minimal

Then restart Thunderbird.

Sidebar tabs
https://www.askvg.com/how-to-enable-new-side-tabs-feature-in-google-chrome-show-tabs-in-left-sidebar/