You'd have to change them from your OS settings haha. Install `gnome-tweaks`, go to fonts, change the size there. Obviously it will change the font size everywhere, not just firefox see https://imgur.com/hpN1p1U

You can set layout.css.devPixelsPerPx to 1.0 (default is -1) on the about:config page. Adjust its value in 0.1 or 0.05 steps (1.1 or 0.9) until icons or text looks right.

• modifying layout.css.devPixelsPerPx affects user interface and web pages (global zoom)

You can open the about:config page via the location/address bar. You can accept the warning and click "I accept the risk!" to continue.

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

Firefox 73+ comes with a Zoom section in Options/Preferences to set the default zoom level for web pages.

• Options/Preferences -> General -> Language and Appearance -> Zoom

You can set layout.css.devPixelsPerPx to 1.0 (default is -1) on the about:config page. Adjust its value in 0.1 or 0.05 steps (1.1 or 0.9) until icons or text looks right.

• modifying layout.css.devPixelsPerPx affects user interface and web pages (global zoom)

You can open the about:config page via the location/address bar. You can accept the warning and click "I accept the risk!" to continue.

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

Firefox 73+ comes with a Zoom section in Options/Preferences to set the default zoom level for web pages.

• Options/Preferences -> General -> Language and Appearance -> Zoom

I've seen this answer posted before and this adjustment has absolutely no effect on anything above the webpage window. This advanced setting only affects the the size of the font within the webpage viewing area. It does not change the "Menu font size" nor the "Tab font Size" nor the "Bookmark bar font size". I'm still looking for a way to customize "Firefox's font size", not zooming the viewing window's font size.

You'd have to change them from your OS settings haha. Install `gnome-tweaks`, go to fonts, change the size there. Obviously it will change the font size everywhere, not just firefox see https://imgur.com/hpN1p1U

I have not installed gnome-tweaks b/c I actually have kubuntu and fortunately, adjusting the font size is baked into kubuntu. I've made as many setting changes as I can and nothing affects the Firefox's font size.

I was hoping to find a setting in Firefox's "Customize..." feature to change, but I guess not.

Perhaps a future version can add a "font-widget" in the "Customize..." manipulations. Or at the very least, add a few new parameters to about:config for users to adjust: font.size.menu, font.size.tab, font.size.toolbar.bookmark, font.size.toolbar.address, font.size.windowtitle

The layout.css.devPixelsPerPx pref affects both the user interface area and the web page area.

Didn't you look at this pref on the about:config page ?

I mentioned the Zoom setting, so you can adjust the page zoom to compensate for the enlargement made by modifying layout.css.devPixelsPerPx.