Hi ineuw, Thanks for sticking with it. I tried doing that... Firstly, moved the chrome folder (with the userChrome.css file being its sole occupant) into each of those three sub folders of Profiles. No change in Firefox. Then I extracted the file from the chrome folder and put it into each of the three folders as well. Well why not as I'm trying all options? To no avail again.

Tiny Fonts are winning... Thanks once more, Steve.

It taught us both something. First, I am a lousy teacher . . . I forgot to ask or look, but I think you are using Firefox in a Mac, and I assumed Windows.

Second, Keep on searching with a more specific search criteria.

Hi ineuw, Another screenshot for you to see that I have placed it into e7xui... (which by the way was only highlighted because I had selected it - it could have been the other two just as easily. But as I said, I've tried it in all. I do note that the file does not appear to have any suffix attached (.css) but it is a matching icon to other files floating around in the sub directory. Could this be anything?

Thanks again, Steve.

Hi ineau, No dramas. In my first post I stated that I used a Mac. You could tell really? So used to it just working without tinkering!

I think I shall have to 'park' Firefox and keep using Chrome until developers find enough interest for enlarging the Tiny Fonts on the toolbar tabs.

You have been an excellent support and took the time to help. Thanks, Steve.

yw and I contributed nothing. The key lies in being familiar with Mac folder, subfolder and file structures and understand what the Firefox profile files are.

Hello, after trying many times I found a solution at least for Ubuntu 20.04 but other operative systems can also follow:

• Locate and open the profile directory:

Go to Firefox menu > Help > Troubleshooting Information > Application Basics > Profile Directory > Click on Open Directory button

• Create the directory with name (if does not exist):

chrome

• Inside create the file (if does not exist):

userChrome.css

Beware if your OS puts a different file extension!

• As the cor-el's answer write in the file:

/* new code wrapper required for FF 7xxx */ 
@-moz-document url(chrome://browser/content/browser.xul), url(chrome://browser/content/browser.xhtml) { 

/*
 * Do not remove the @namespace line -- it's required for correct functioning
 */
@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* set default namespace to XUL */

    *|*:root * { /*your font rules*/
       font-size:16px !important;
       text-shadow:none !important;
    }
}

PS: If you want bigger font just change the font-size number:

16px

• Now we need to allow Firefox to read the file:

Open a tab and write in the address bar:

about:config

In the search box write:

userprof

If you find:

toolkit.legacyUserProfileCustomizations.stylesheets

Having the value false then double click it to turn it to true.

• Finally you only need to restart Firefox.

• PS: This solution has a little problem that extensions that show some helper text or numbers turns the font to be the same side has the other UI, so if someone know how to solve it in the css please respond.