Version 46.5 works with Firefox ESR 52.5.3

Download the file, drag it onto the addons screen, confirm install prompt

https://addons.mozilla.org/firefox/downloads/file/438176/type:attachment/theme_font_size_changer-46.5-tb+fx+sm-mac.xpi?src=version-history

Make sure you have disabled automatic updates for the addon (double click it at "Add-ons Manager" screen, scroll down, select "Off" for "Automatic Updates")

This is an awesome program that makes the unreadable font in fire fox readable. I would rate this app with 5 stars, since the author decided to be shady and add an expiration date i can only rate it at a TWO. Thanks to everyone who made a work around for this amazing app shame the author is shady though. I wish they would allowed us to continue enjoying this app.

@Hackodis
I previously rated it at 5 stars; wish I could give it negative now.
Mozilla ought to pull this.
57 does not work with FF 57.
56 no longer works with FF 56.
Maybe a class action lawsuit by us folks with bad eyesight ...

Anyone with a way to make the add on work in a Mac OS (10.11)? Everything posted here and elsewhere is Windows.

Main thing I need is the bold.

I'd gladly pay for an app or utility that does what 56 did - or pay Boris ...

It won't install for Thunderbird and there is where I need it.

Anonymous user cf029c - What version of Firefox are you using? I'm using 56.0. Your method doesn't work.

not working

Whatever you changed, it's still not working. No Icon can be found on Firefox nor Thunderbird to launch the App.

It just stopped working for firefox and thunderbird yesterday (Fri, Jan5). The text in the view is so smaller I cannot read it, especially in Thunderbird

Can't live without theme font size changer! Please fix it so that it works with FB and FF again! Thanks!

@GVP

Thanks for the hint but the site says "This is effectively an unsigned version and thus cannot be installed for Firefox" and Firefox really refuses to install it, any other idea ?

I do not use Thunderbird and would only need it for Firefox but see above

Well, I just uninstalled the addon and now Firefox seems to use the general Windows font settings I set longer time ago, rather too big in some places (address bar etc.) but necessary in some other programs very probably and better than too small anyway so maybe this will be a solution for some other people, too

I've seen your Theme and fonts resize has been updated today (6th January 2018) but still not working :-(
Please fix it asap.
Many thanks

I found a workaround for the loss of TFSC that works on Firefox.

1. Create a "userChrome.css" text file: https://www.howtogeek.com/334716/how-to-customize-firefoxs-user-interface-with-userchrome.css/

2. Add the code found here for "Global UI font": http://kb.mozillazine.org/Pane_and_menu_fonts
Change parameters as desired.

3. For bold font add this line before the end symbol: font-weight: bold ! important;
Change parameters as desired.

4. Save file and restart Firefox. There may be other parameters available but these were all I needed,

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