One other thing that Safe Mode does is disable hardware acceleration. It's not obvious (to me at least) why this should affect font sizes.

You can turn it off and on at will here:

Tools|Options|Advanced|General

Thanks Zenos.

I've turned it off but it doesn't alter anything.

Just to add, this is a portable version of TB which I have since tried on another computer to make sure it's nothing to do with a specific PC or the fonts installed (not that I though it would make much difference) and it still does the same thing.

If the add-on doesn't work (and its support for Linux has been discontinued, so some of us are forced to find other ways of doing it) then another way to go about it is to create a userChrome.css file, with the following contents:

/*
 * 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;
  }

I chose the font size and face to match those used elsewhere in the Windows GUI. You could change or omit the font-family, and you'll need to change the font-size to suit your needs.

You'll need to find your profile, create a folder in it named chrome, and create a text file named userChrome.css with the contents as given above.

Similar content in another file named userContent.css located in the same folder can be used to set the fonts used to display the content of your messages.

Thanks.

Unfortunately I'm using this at work where I'm forced to use Windows. I use another instance of TB on my home Linux box, but I digress...

Although the CSS file would no doubt set the font size and family, it doesn't explain why the settings in options don't work or why the default font settings don't apply when disabling the Extensions. I'm pretty sure (although I may be proven wrong) that it has something to do with Theme Font Size changer thing... not being removed properly, I just can't find the files to remove manually. I know this may suggest they have already been removed but I'd like to be sure.

Thanks for working on this Zenos, much appreciated.

Okay, I had a userChrome.css file and it contained font settings already...font-size was 8px (ooops...) !

I must have done this last time I had problems to test changing the font size but then updated the Extension and forgot all about it! Doh!!

Thanks for the advice and help Zenos, you fixed the problem :)

Hmm, we get a fair number of queries about the font sizes (etc) in the menus and message lists. And we pretty much always advise to try the Theme Font & Size Changer add-on. So I am not so sure that "default font" is a useful concept in Thunderbird. It doesn't seem to naturally track what's going on elsewhere in the OS/GUI.

Deleting an add-on doesn't ordinarily remove its settings. I think this is deliberate, so that if you resume using an add-on, it picks up where you left off, and don't have to set up from scratch.

I'd just start working through the extensions folder in the profile, opening the install.rdf files and looking for the <em:name> entry. The search in files feature of Notepad++ can do this all for you.

Like you, I (have to) use Windows at work and choose to use Linux at home. Having found a solution for the Linux case, it seems natural to use that same solution here in Windows, rather than rely on an add-on that might not always be available.

I've commented out the font settings in the css file and restarted. The font remains "fixed" as in I can read it now but I can't change the font in Options > Display > Default Font.

It seems all still is not well.

I've been through the extensions folder but found nothing relating to Theme Font & Size Changer, in-fact the only thing I have found is a folder called themefontsizechangerrainbow-easel in my profile directory which contains a db file called themefontsizechangerrainboq-colors.sqlite.

Options > Display > Default Font is for message content font. Theoretically, it shouldn't affect GUI font used for menus, folders, message lists etc.

The very very tiny fault happened TODAY on my Windows 10 Thunderbird. Message fonts are fine (Calibri 17), it is only the Main Screen (listing folders and messages) that suddenly became very small.

I am looking for a solution that does not involve modify css files or whatever, just an option to set fond for main menu like works for messages.

I have the exact same issue on the MAC version, I don't begin to know where the "profile" even is so I have no way to add a folder or anything else for that matter. This should be so simple and very much a standard part of the interface, why is this such a mess now?