Hi, that would be correct as Firefox Security is blocking it from interfering and changing the program it does not have permission to be changing. The company would have to make a add-on or get permissions granted after testing for this to be allowed. It is up to the company to do this.

As for https://addons.mozilla.org/en-US/firefox/addon/theme-font-size-changer/ it works on versions 57+ You do not tell us your version in your question not does the info show.

Firefox also has it's own text zoom and it can be found by going to the 3 Bar Menu --> Customize and dragging the -- |100% | + ot to the bar for use.

Do you have the latest updates? There are references in the release notes to fixes for Firefox 59+:

See "ZoomText 2018 May Update (v2018.1805.53)" in https://www.zoomtext.com/help/releasenotes/

Font Size

Firefox has a setting to globally enlarge all text and images, including both the toolbar area and the content area. I'll put the steps toward the end of this post in case you think it would be useful.

Font Face and Weight

I don't think extensions can change this yet. Instead, you might consider using custom style rules in a userChrome.css file. Perhaps someone already has put together the code needed, or someone might need to create that for you. The most essential ingredient would be knowing which font on your system you want to use instead of whatever Firefox's standard selection is.

Global Zoom Factor

By default, Firefox follows your system's "Text Size" or DPI setting. That may be 100%, 125%, or on a Retina display 200%. Firefox also lets you override this and specify your own scaling factor.

This will scale up both the text and the icons in the toolbar area, and in web pages. Here's how to give it a try:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful or accepting the risk.

(2) In the search box above the list, type or paste devp and pause while the list is filtered

(3) Double-click the layout.css.devPixelsPerPx preference and:

• Mac with Retina display: enter 3 and click OK. This is equivalent to 150% on a Retina display.

• Otherwise: enter 1.5 and click OK. This is equivalent to setting the "Text Size" in Windows 7 to 150%.

What do you think, is this the same, better, too large? You can experiment with this setting, but I suggest not making it smaller than 1 and not making it larger than 4.

If things get out of control, you can right-click (on Mac, Ctrl+click) the setting and choose Reset, which is the bottom item on the right-click context menu.

Can you find a size that works well overall?