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[SOLVED] Mozilla Thunderbird Theme Font & Size Changer add-on suddenly did’nt work. Here is the solution

The mail client Thunderbird depends on the add-on Theme Font & Size Changer. Without this add-on font sizes in the interface (message list, menues, folders, dialog boxes) are getting very, very small. Most people won’t be able to read and use Thunderbird without this extension. I have no idea why the extension is needed at all, but it is.

Suddenly January 5, 2018, this add-on stopped working. For some reason you could not set the font size, and the add-on button next to the menues disappeared. There has been a lot of debate about this, because Thunderbird (and Firefox) is used by many people.

Here is a solution:


write ‘about:config’ in the location bar. Accept the warning. In the top bar search for ‘pix’. In the list coming up double click on layout.css.devPixelsPerPx. Change the value to 2 (it is probably allready minus 1). You can change it to whatever positive number. This will make the fonts in Firefox larger.

Thunderbird Mail

Thunderbird has the same problem. Download the Theme Font & Size Changer. I used version 56, but version 64 might go as well. If you allready have the theme installed please remove it.

The add-on is an xpi file, which is very much the same as zip. Unzip the xpi file. Find the file Bootstrap.js and open this file in your editor. You now need to find a specific code line. Its usually line number 1382. (you can also search for it)

Save the bootstrap.js file. Now you need to zip all files again. You need to mark all the files you have unzipped (opposite going one level higher and zip the folder containing all the files – wrong). Mark the files (bootstrap.js, chrome.manifest, install.rdf, chrome, default m.v.) and zip them all together to one zip file.

Change extension to xpi and rename the file to whatever you want. It could be MyThemeFont.xpi

Now – go to the add-on window in thunderbird and import this MyThemeFont.xpi file. Now – I hope – you should have a button labeled “Theme Font & Size Changer” in the upper right corner in Thunderbird. Push the button. In the third field change the size to – let’s say – 18,0. And it should work.

Next time there is an upgrade of either Thunderbird, Firefox or this add-on you might have to repeat everything again.

Please make a link to this page from your website. Thanks.