Theme Font Size Changer

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Ken Saunders

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Post Posted July 19th, 2010, 11:54 pm

AMO Listing

Theme Font Size Changer allows you to adjust the global font size for Firefox itself (meaning all windows, menus, toolbars, etc). It was developed primarily as an accessibility aid for persons with poor eyesight and visual impairments, but it can be greatly beneficial for Firefox users with wide screens and high resolution settings, and for other users who find the default font size for Firefox to be too small.


There are 4 ways to access Theme Font Size Changer's menu to choose a font size.
1. Toolbar button (optional, not applied by default)
2. Status bar button (right or left click)
3. Tools > Theme Font Size Changer
4. Tools > Add-ons Manager > Theme Font Size Changer > Options

Select a font size value (from 9 to 40 pixels) and it will be applied instantly and globally.

Theme Font Size Changer was designed for use with Firefox's default theme, but it does work with some non-default themes. Just please keep in mind that the fact that it works with non-default themes is more of a bonus than the primary goal and function of it.

Compatibility issues found when tested with 32 Firefox themes were minor overall, a few were major, and some had no compatibility issues at all.
Any issues will mostly be dependent on the theme (or extension) developer's CSS value choices for any given element.
Theme Font Size Changer will not change the font size in cases where custom (non-default) font sizes are used with the !important rule.

Known issues with some non-default themes.

The font size in some elements in non-default themes remains unchanged.
For example and through testing, the font size of tab text was unchanged by Theme Font Size Changer in some themes.

Repeating backgrounds.
With some non-default themes you may encounter repeating backgrounds (on toolbars, etc) when the font size is increased. Increasing the font size also increases the height of other elements (like toolbars) and so if specific CSS properties are not set to prevent backgrounds from repeating, then a background will repeat itself which you may find to be undesirable.

Text bleeding out of elements or being cut off.
You may notice this in some themes such as on dialog buttons, drop down menus, and tabs.

Theme Font Size Changer will work in Minefield if you bump up the max version number, but there are a few minor Firefox 4 specific issues (Add-ons Manager sidebar, and toolbar buttons backgrounds). They are Firefox issues, not Theme Font Size Changer ones.

Using your own CSS rules (with Stylish, etc) will resolve most of the issues noted.
For example you could use the following to fix the tab text one.
.tab-text {
font-size: 30px !important;

No known compatibility issues with the following themes.
NASA Night Launch
Kempelton Large
Mostly Crystal
Walnut for Firefox (except for status bar text)
Ambient Fox Xyan
Themes by Aaron Spuler

If you're a theme author, or know of other themes that Theme Font Size Changer works with without issues, please leave a comment so that I can add it to the list here and on AMO.

There are no known compatibility issues with Personas.

Example of Theme Font Size Changer with a non-default theme.

Get Theme Font Size Changer
Overview on Access Firefox (includes more screen shots)

Thanks for your time.

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Joined: February 23rd, 2009, 7:04 pm

Post Posted January 24th, 2014, 12:34 pm

How can I change the colours on the Theme and Font Size changer. I found the colour chart and changed to dark black, but the background in some windows is grey and I want it to be plain white. It seems strange that the Theme and Font Size Changer is aimed at persons with sight deficiencies, but it offers no colour options that I can see.


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Post Posted December 20th, 2015, 3:58 am

My thanks to the dev for updating this. As fr as I know, it's now working fine on the two versions I use: Nightly (Fx 47) and Beta (45).
Last edited by joeg on February 11th, 2016, 1:57 am, edited 1 time in total.
Although every day is Judgment Day, I nonetheless feel like a room without a roof.

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Joined: January 13th, 2012, 11:29 am
Location: Baton Rouge, LA, US

Post Posted February 10th, 2016, 5:29 pm

This is a great add-on/extension. Unfortunately currently it won't install on my Firefox 44.0 as it is 'not available for this platform'. Platform is Firefox 44.0 in openMandriva and/or openSUSE. These are Linux operating systems. It used to work until as far as I know today. It does work in Thunderbird.

Is there any way to get this working again in Linux for Firefox?

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Joined: January 13th, 2012, 11:29 am
Location: Baton Rouge, LA, US

Post Posted February 11th, 2016, 5:55 pm

As a work around for Linux users see this post:


Working for me in Firefox 44.0 in OpenMandriva Lx 3 Alpha. I'm so happy to be able to see now. :shock:

You can change font and font size to suit your needs. :D

It's post 13 at the end of thread. Based on Old Bozz post number 6 in same thread. Thanks Old Bozz. =D>

I really hope 'Theme font and size changer' can be made to work in Linux again. :cry:
Ben Bullard

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Joined: September 29th, 2017, 7:15 am

Post Posted September 29th, 2017, 7:21 am

the Theme Font Size Changer is a great extension to firefox and thunderbird, which I am using on my Lenovo Yoga Thinkpad.

Since I am using as a tablet as well as my 'Desktop PC' with a larger external monitor, I wouldlike to toogle between a large font (for tablet use) and a normal font (for desktop use). With the tool GUI it takes some mouseclicks to do this .... a button to switch between two predefined profiles would be a perfect feature, which might be addes to the tool i n a future version. Or is there a wokaround to do this right now with the current version?

thx for any help

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