Are we talking about this extension? Or this one?

Firefox prevents me from the two downloads at your first link. I was able to download Baris Derin's 64.1, but no icon appears on the tool bar.

To clarify my last comment, I believe the first link you gave was for Firefox add-ons.

I know of four solutions:

1. use a new version of the add-on
2. bodge the original xpi file to undo the timebomb
3. adjust the layout.css.devPixelsPerPx or layout.css.dpi settings in the Config Editor
4. set up userChrome.css

All of the last three work for me. I haven't tried the fixed variant of the add-on but it seems to me that if you can fix it for your self, one fixed by someone ese will be just as good. So it comes down to a choice as to which solution is most durable or attractive.

Here: https://addons.mozilla.org/en-us/thunderbird/addon/theme-font-size-changer-fixed/ is a new version of the add-on which doesn't have the timebomb. But who's to say when this will become incompatible?

The original add-on is not reliable is it needs to be renewed periodically and the author seems disinclined to do this in a timely manner. Also this add-on is not available to Linux users.

The Config Editor approaches come with caveats that they are not intended for this purpose (so what are they for? I don't know.) One of them is not available to Windows users, and both affect the size of everything in Thunderbird. So you might need to make a second set of adjustments for message text size to undo what this config setting does to them.

The one that works for me, and keeps on working, is the userChrome.css file (and also a similar userContent.css file that applies to message text.) So I'd recommend you stick at making it work.

You need a folder named chrome in your profile. In that folder you need a file named userChrome.css (The spelling and capitalisation of both of these are important!) In that userChrome.css file you need this text:

/*
 * 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 */

/* 
 * Make all the default font sizes 9 pt:
 */
* { 
   font-size: 9pt !important; 
   font-family: "Segoe UI" !important;
  }

The font-family line is optional; I use it to set the message lists and folder pane to use the same font as the rest of my UI. Change the 9pt to whatever suits your needs.

So you need to look carefully at where you might have diverged from this procedure.

Some of the content of the non-message panes in Thunderbird might be affected by the regular settings for message text size, but again you can pre-set these by using the same code above in a different file, in the same chrome folder, named userContent.css.

However, whilst userContent.css can be useful for applying some preferences that don't appear elsewhere (bold font perhaps?) in general I find it better to use Tools|Options|Display|Formatting→Advanced to set the message text appearance.

lnafau said

To clarify my last comment, I believe the first link you gave was for Firefox add-ons.

But it says this:

Theme Font & Size Changer for ThunderBird Version History

So you believe wrongly.

Thunderbird and Firefox use the same mechanism for add-ons. So any add-on for Thunderbird could be seen by Firefox as a Firefox add-on, so you have to take care to download it for installation into Thunderbird, and not allow Firefox to try to install it. Right-click is your friend.

Zenos, thank you for your replies and the time you have taken to help me with this issue. I will go through your suggestions and see if I can make any headway. Had to google "Bodge", although I had a rough idea what it meant. ;-)

Success at last! When originally building the "userChrome.css" file I had unknowingly saved it as userChrome.css.txt. Got rid of the .txt and all is good. (Bob's your uncle?)

Thanks again for the help Zenos.

Windows' Notepad does that too. A subtle trap for the unwary.

It should always be possible to go back afterwards to check and change the filename if necessary, via your file manager.

I guess "kludge" might be a more internationally acceptable word than "bodge", but I'm not entirely sure that it can be used as a transitive verb.

Bodge works for me.

I did use notepad to build the file. Lessons learned in this exercise. Keep up the good work.

Thanks for the input. Not sure if I have the skills for this, but will study it and attempt when I feel confident. On another matter, I have downloaded and installed both the Firefox and TB versions to their respective programs. Everything was fine until the latest update. Makes one wonder if updates are always good. Thanks again

I have revised the answer posted above. Do try the "fixed" version of the add-on.

https://addons.mozilla.org/en-us/thunderbird/addon/theme-font-size-changer-fixed/

FIXED VERSION WORKS GREAT!!!!! THANKS SOOOO MUCH.