To increase all the fonts, see this topic about changing a single preference:

https://support.mozilla.org/en-US/questions/1269361

The discussion includes a method to adjust just the message list (Threads Pane font) or the Folder Pane with css:

https://support.mozilla.org/en-US/questions/1269361?page=2#answer-1274848

Dear sfhowes,

Thanks for the information. I tried the Config editor approach, changing "layout.css.devPixelsPerPx" to a larger value. That helped only slightly and larger numbers have no effect. I'm on a Macbook for what it's worth.

There is another method to put a bit of CSS into a subfolder, but I haven't tried it yet. The direction is to call the subfolder "chrome," so I was wondering if that was application specific to the Chrome browser, though I don't know why that should be. . . . ?

Chrome, in this case, refers to styles, and is not related to the Chrome browser, and predates it by several years. A css file in a chrome subfolder is also used with Firefox, which shares a code base with TB.

Thanks! I'll apply the CSS and see how it works and let you know.

If you're using TB 78, you will have to double-click toolkit.legacyUserProfileCustomizations.stylesheets to true in Config. editor, under Preferences/General.

I've done all the above, but the text in both the sidebar and the message list refuses to get bigger. The text in the body of the emails is huge, but I can go back into prefs and reduce them. Thanks anyway. It's as if there's something in Catalina that is over-riding the app preferences for those particular text areas. Either that, or TB doesn't want me to change them. . .

There are several pitfalls in applying css: the folder must be named chrome, and it must be a subfolder of the profile folder; the file in chrome must be named userChrome.css and be a css (Cascading Style Sheet) file type, not plain text, and the preference must be switched to true. If you select Help/Troubleshooting, click Show in Finder, the profile folder will open, so you can check that chrome and userChrome.css are in the right place.

The problem I see with both of the proposed resolutions is that they only address part of problem. The font height changes but the line spacing is spread too much. The .css line-height property doesn't seem to be effective (see spaces identified by the arrows in the uploaded image).

If you're using TB 78, you will have to double-click toolkit.legacyUserProfileCustomizations.stylesheets to true in Config. editor, under Preferences/General.

So....where is this most important file toolkit.legacyUserProfileCustomizations.stylesheets? Did a find on the mac, no results.

If you're going to post file names, post the entire path.

sfhowes said

If you're using TB 78, you will have to double-click toolkit.legacyUserProfileCustomizations.stylesheets to true in Config. editor, under Preferences/General.

So....where is this most important file toolkit.legacyUserProfileCustomizations.stylesheets? Did a find on the mac, no results.

If you're going to post file names, post the entire path.

It's not a file, it's a preference in the TB Config. editor. Open Preferences in TB, type config in the search.