See https://support.mozilla.org/en-US/questions/1360794

Thanks, this seems to be a solution, but how do I implement it? I am not a code writer or programmer. I can't even get started with the given steps.

For instance, what does "Help/More Troubleshooting" mean? I clicked on the Help menu but can't find a "More Troubleshooting" item. See below:

I have the steps down as the following. Correct? 1 Help/More Troubleshooting, (?) 2 Profile Folder, 3 Open Directory, 4 Close TB, 5 Create a new folder named chrome, 6 Create a new document in chrome with a text editor, 7 Name it userChrome.css, 8 Save as type: All files *.*, 9 Copy in the above style sheet code, 10 Change the “font-size:” numbers as desired. (My note: try “14pt”) 11 Double-click toolkit.legacyUserProfileCustomizations.stylesheets to true in Preferences/General/Config. editor, 12 Restart TB.

Help/More Troubleshooting is the menu in TB 91, the current release. In older versions, select Help/Troubleshooting Information. Instead of Open Directory, it's Show in Finder on OS X.

The topic cited in my first reply also includes a simple method to zoom all fonts.

Thanks. I thought I was running the latest version of TB -- 68.12.1? How do I get TB 91?

Thanks sfhowes. I'm running an older version of OS X, so TB 69.12.1 is the latest for me.

I see two options for "Show in Finder". Which do I show. 1. Root Directory 2. Local Directory (see screen shot)

Choose the Root Directory if you've opened about:profiles, or Profile Folder, Show in Finder, from the Help/Troubleshooting Information tab.

To update from 69 to 91, first install 78, then install 91.

https://archive.mozilla.org/pub/thunderbird/releases/78.14.0/mac/

https://archive.mozilla.org/pub/thunderbird/releases/91.4.0/mac/

Export your address books to LDIF format before updating in case the process isn't perfect.

Thanks sfhowes. I chose the Root Directory and opened it in the Finder, folder "2veujpsr.default". 1. I created a new folder "chrome" within the 2veujpsr.default folder. 2. In a text editor, I created a new blank document to later place the style sheet code into and named it "userChrome.css". But when I went to save the document so I could place it into the "chrome" folder, it saved the document as "userChrome.css.rtf". Is this ok? 3. I copied the style sheet code into this document. (see screenshot below). What does the step "Save as type: All files *.*," mean?

No, you don't want to save it as an rtf file. On Windows, All files *.* saves it in the format specified by the extension, .css (Cascading Style Sheet), if your text editor doesn't offer the choice of saving as a .css file type.

Try saving as plain text .txt and then changing the extension to .css.

https://apple.stackexchange.com/questions/50055/how-to-save-a-css-file-from-textedit-or-word

Ok, thanks sfhowes. Have created a .css file for the style sheet code.

Question: Is the new "chrome" folder supposed to be placed in the "Profile" folder or in within the "2veujpsr.default" folder that's already in the "Profile" folder?

See screenshot for how I have it set up right now:

The chrome folder must be in the 2veujpsr.default folder.