This procedure will change the colour of the active tab in Firefox(v59+) or
Thunderbird(v60+). The procedure is essentially the same for both Firefox and
Thunderbird.
Make sure that your "file explorer" is configured to show "hidden folders". (top
menu, file, view, select "Hidden items on the right)
First - Ensure that you have a theme (Dark or Light) selected.
This procedure has no effect on the Default Theme!
For Firefox v69+ you need to enable userChrome.css.
about:config and set "toolkit.legacyUserProfileCustomizations.stylesheets" to
true
Go to your
(For Firefox) C:\Users\ your user name
\AppData\Roaming\Mozilla\Firefox\Profiles\ DIRectory
(For Thunderbird) C:\Users\ your user name
\AppData\Roaming\Thunderbird\Profiles\ DIRectory
Open your profile folder. If there is more than one profile use the one with the
latest date.(should be something.default or default-release ie
fhty6gbo.default.release)
Look for a folder named chrome - if it does not exist create it
Open the chrome folder
Using Notepad or similar editor create a file named userChrome.css
Add cut/paste the following lines of text to userChrome.css
Note: Leave the @namespace line as is. Required for correct operation.