I too was having the same problem as many others have posted about (extension stops working and no icon in toolbar even when installing old verisons). I was able to fix it by installing an even older one, Version 46.5. There still is no icon to click, but you can access it from the Tools menu in Firefox. It seems to work fine now, just make sure to disable automatic updates!

I've used this add for years and in general it is a godsend.
Periodically however it just falls over.
Last night my fonts reverted to their microscopic version.
I checked and found a new release.
As in the past I remove all old versions and then install per the instructions.

For TB the TF&S change should be visible in 2 places.
I have zero.
I am showing V62 as enabled and installed but that's all.
As said a truly great add on that really should be a part of TB BUT ONLY WHEN IT WORKS.

Not enjoying it one bit today! All of us have spent hours trying to fix this font issue .... I'm not a programmer and not comfortable with the work-around offered. Thunderbird--big fix for all, please!!

I am using this addon in thunderbird too and there is a fix available and it works but only for thunderbird where I applied it without problem. Please see:
https://addons.mozilla.org/en-US/firefox/addon/theme-font-size-changer-for-tb/reviews/
It is not possible to install adjusted addon back to the firefox maybe due to signing addons which is required by firefox.

But there is another important aspect: the creator of this addon implemented time check into source code which was needed to remove
to ensure proper function in Thunderbird. Please see details in forum link I posted above.
Many thanks mrx2020, HackerKitty, P. Maigron for spotting this. And of course the same "time check" is in source code for firefox too. I do not judge anybody just inform abou the issue which prevents me from using firefox and thunderbird properly.

Edit: posible workaround for firefox thanks Eckard :
http://forums.mozillazine.org/viewtopic.php?f=38&t=3035148
my userChrome.css:
@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");
@namespace html url("http://www.w3.org/1999/xhtml");
@namespace svg url("http://www.w3.org/2000/svg");
* {
font-size: 14px!important;
font-family:Comic Sans MS !important;
}

great addon but it broke FF56.2 today.
not everybody cares about mozillas FF57. If I want fast I can install chrome no need for FF57 MeToo
I will never install FF57 so pls fix your app for FF56.2 and leave it alone

FF57 won't be a success so no worries for you.
If everything breaks I will move to Pale Moon

Downloaded the TFSC date code fix that was detailed by "Anonymous user cf029c" and I can see menus and bookmarks in FF again. Thank you! Your instructions were clear and easy to follow.

Baris, I am on the latest version of Firefox, using your latest version of this add-on...yet I have the problem too. It isn't functioning for me at all, so something is definitely wrong, here.

Thank you for the tip 46.5 version works I can see again

reverted to ver 46.5 on thunderbird all now working hope it stays that way

I set firefox and addons to not update past 56.1 but now Theme Font & Size Changer is not working on any of my computers.

Oh, come on. What now? Please do fix this! Old versions no longer work...

Alles war perfekt funktioniert nicht mit FF 55.0.3

What happened with this add on, today? On every pc in my family, 7 computers total, TFSC stopped working. No updates were installed. Automatic updates are turned off. Every computer hast FF ESR 52.5.3 64 bit installed.

Did everyone get a "deactivate" signal from an external source??? Am I the only one with this theory?

So this guy Berin or Gerin wants access to all data for every website I visit in exchange for the use of this add-on? This guy has lost his mind. I'd rather buy it (one time) with no access to him for anything.

Helpful info.. (I've been here before because of nonworking TFSC) and my old eyes need help, like a lot of peeps showing up here :)
First, I only use PC Waterfox 64bit 54ver. and I use Thunderbird 32bit 52ver. I have ALL my addon's turned off and not to update including WF and TB. I realize now, That Firefox and Thunderbird will never add some helpful font adjustments for our bad eyes :(
So,...
If you read through the reviews. You'll see some info reviews on how to download and install a non expiring version of TFSC on earlier version of FF, Waterfox, TB. I only wanted to help people because I feel their pain on not being able to see and need help and remember to turn off all updates that includes FF, TB, WF. . I still want to give props to the Author of TFSC. Thx you!
.

it did work fine until today..... suddenly i can´t access it anymore and everything is small again.
deinstalled and reinstalled the addon but it does not appear in thunderbird 52.5.2.


don´t know if mozilla messed around again... can only say i is not working anymore. hope it will be fixed soon.
i thought the web extension stuff ony affects firefox 57??

Why the Version 56 even on Firefox 56.02 stop working !! why you do that ???? the add_on working great until this afternoon !

CAN YOU PLEASE FIX IT !

Ich finde das add-on Spitze auf Thunderbird. Aber leider scheint es auf meinen Firefox ESR 52.5.3 nicht zu funktionieren? Schade. Warum ist dies so?

Great addon firefox menus are too small to see without it.

This addon shouldn't exist there should just be a firefox option to increase menu font size but there's not, and this addon filled the gaping usability design flaw left by mozilla.

But Firefox killed it. : (

In firefox's constant quest to make a crappier version with less features and capabilities every revision, they made it so this addon will no longer function for safety reasons.

Thanks Mozilla I'm safer but now I can't see the menus. >:(

good

this sucks - there is no where to change the size of the text anywhere - so damn frustrating.....FUCK!!!

I just updated Firefox and I can't change my Theme back to what I had it on.... It's says 'disabled' and when I try to enable it, nothing happens at all... How can I fix this??