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/

To have better visibility of labels of mails in the mail list, the style can be changed by closing Thunderbird, editing or creating a chrome/userChrome.css file in the Thunderbird profile directory, then starting Thunderbird again.

userChrome.css

/*
 * Label colors.  Highlight background rather than change font color.
 */

/* Default Important Label */
treechildren::-moz-tree-cell(lc-FF0000) {
    border-bottom: 1px solid #FF0000 !important; background-color: #FFCCCC !important;
}
treechildren::-moz-tree-cell-text(lc-FF0000) {
    color: #000000 !important;
}
treechildren::-moz-tree-cell(lc-FF0000, selected) {
    background-color: #FF0000 !important;
}
treechildren::-moz-tree-cell-text(lc-FF0000, selected) {
    color: #FFFFFF !important;
}

/* Default Work Label */
treechildren::-moz-tree-cell(lc-FF9900) {
    border-bottom: 1px solid #FF9900 !important;
    background-color: #FFCC99 !important;}
treechildren::-moz-tree-cell-text(lc-FF9900) {
    color: #000000 !important;
}
treechildren::-moz-tree-cell(lc-FF9900, selected) {
    background-color: #FF9900 !important;
}
treechildren::-moz-tree-cell-text(lc-FF9900, selected) {
    color: #FFFFFF !important;
}

/* Default Person Label */
treechildren::-moz-tree-cell(lc-009900) {
    border-bottom: 1px solid #009900 !important;
    background-color: #99FF99 !important;}
treechildren::-moz-tree-cell-text(lc-009900) {
    color: #000000 !important;
}
treechildren::-moz-tree-cell(lc-009900, selected) {
    background-color: #009900 !important;
}
treechildren::-moz-tree-cell-text(lc-009900, selected) {
    color: #FFFFFF !important;
}

/* Default Todo Label */
treechildren::-moz-tree-cell(lc-3333FF) {
    border-bottom: 1px solid #3333FF !important;
    background-color: #CCCCFF !important;}
treechildren::-moz-tree-cell-text(lc-3333FF) {
    color: #000000 !important;
}
treechildren::-moz-tree-cell(lc-3333FF, selected) {
    background-color: #3333FF !important;
}
treechildren::-moz-tree-cell-text(lc-3333FF, selected) {
    color: #FFFFFF !important;
}

/* Default Later Label */
treechildren::-moz-tree-cell(lc-993399) {
    border-bottom: 1px solid #993399 !important;
    background-color: #FFCCFF !important;}
treechildren::-moz-tree-cell-text(lc-993399) {
    color: #000000 !important;
}
treechildren::-moz-tree-cell(lc-993399, selected) {
    background-color: #993399 !important;
}
treechildren::-moz-tree-cell-text(lc-993399, selected) {
    color: #FFFFFF !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;}

Sidebar tabs

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