Go to the add on authors page and read his solutions. If you have questions about add-ons ask the author. They are responsible for supporting their software.

I've been to the add on pages an found no solution. After further reading it seems Mozilla no longer supports Thunderbird so apparently I asked my question in the wrong place. Sorry for the intrusion. Just thought the Firefox experts would know the answer to my question as to which older version might work.

http://www.rossde.com/ThemeFontSizeChangerNoexpire/index.html

This is still the official place for Thunderbird support.

The issue is not due to any Thunderbird or Firefox versions but due to the author having put in a datebomb expiration that made the extension stop working on January 5th.

I just read the comments on the authors site yesterday and there were fixes discussed.

Airmail said

I just read the comments on the authors site yesterday and there were fixes discussed.

Guess I am just a lowly user - those fixes went right over my head. Logically it would seem to me that there should be no need for an add on if the issue had been addressed years ago. Again sorry for the intrusion, I shall move on now.

The author's site unhelpfully refers Thunderbird users to a firefox site:

https://addons.mozilla.org/en-US/firefox/addon/theme-font-size-changer-for-tb/

However if you change that firefox to thunderbird, you get here:

https://addons.mozilla.org/en-US/thunderbird/addon/theme-font-size-changer-for-tb/

I haven't yet found for myself any helpful explanation there. Care to elucidate, Airmail?

There is a user comment that refers back to this site, recommending the userChrome.css method. Interestingly, it points to a posting almost two years old, and not related to the current flurry of unhappy TF&SC addon users.

Did the modified add-on mentioned above not work for you? What problems did you encounter?

The post from Happyone has a link to Google drive. Several people are thanking him for the fix. So I would take that as a workable fix. I do not use this add on and have no desire to install it.

I have to agree with most people using the add-on that this should be a standard adjustment built into Thunderbird.

Okay, I found a workaround that works for my wife - and that's all that matters - by changing the display settings to 'custom' and chose 125%. Of course that messes with desktop icons which become bigger and need to be re-arranged to her liking. The Thunderbird inbox is now readable again which was the objective. We are running Windows 10 and display settings are found by right clicking on the desktop, Choose 'Display settings', scroll down to 'Scale and Layout', click on 'Custom Scaling' then put in desired number. I tried both 150 and 125 but settled on 125 which seemed to be less disruptive. At this point you need to sign out and back into Windows for the change to take effect. Hope this helps someone else.

IMHO, that's a poor fix. You should use that setting to get Windows to play nicely. At both work and at home I need that to be set to 125% to be able to read Windows' menus and dialogues. In Linux it's is an honest and straightforward setting for the DPI (144 and 96 in my case) which you can find from the specifications, or work out from the logical and physical dimensions of your display. Both Apple and Microsoft seem to think this is too "technical" and take away from the user this simple ability to set things to their correct nominal values.

Having done that, I use the userChrome.css file method to set both Thunderbird and Firefox interfaces so they use the same font and size as the rest of the UI. Having got the DPI right, this is more for consistency and aesthetic considerations than necessity.

The userChrome.css file is likely the best solution but when I see the following it tends to deter a user from pursuing that 'fix':

"https://developer.mozilla.org/en-US/docs/Mozilla/.../Modifying_the_Default_SkinNov 17, 2017 - This documentation has not been updated for Firefox Quantum. Support for the userChrome.css file and any of its elements described below are not guaranteed in future versions of Firefox. Using it may lead to hard-to-diagnose bugs or crashes. Use at your own risk!"

So far the adjustment to display settings mentioned previously seems to be working okay. In fact when I checked that setting on my laptop, which is rarely used for email, I see it is set to 125%. I must have made that change from the default years ago and forgot about it.

Guess that's what's great (and sometimes frustrating) about computers - there's usually more than one way to accomplish the same thing.

But that won't apply to Thunderbird.