Hi,

You can try to set a Minimum font size in Tools (Alt + T) > Preferences > Content > Advanced (under Fonts & Colors).

Options > Content

Tools > Options

Theme Font & Size Changer extension isn't intended to change the font size for web page content, that's what Page Zoom of Text Zoom is used for.
https://addons.mozilla.org/en-US/firefox/addon/theme-font-size-changer/
About this Add-on

" Different from add-ons that enlarge and reduce text on web pages, Theme Font & Size Changer changes the font size and font family in all windows, menus, toolbars, etc within Firefox itself. All font size and font family changes are instantly applied and they are saved permanently and through sessions. You may assign your favorite font into your browser interface and change its size as you wish."

The Firefox Page Zoom feature does a domain by domain level of saving the users preferred zoom level settings, there is no default Page Zoom level setting in Firefox, as with some other browsers.

Try the Default FullZoom Level extension and set a default zoom level:
https://addons.mozilla.org/en-US/firefox/addon/6965

Or try the NoSquint extension:
https://addons.mozilla.org/en-US/firefox/addon/2592/

thank you everyone for posting answers. under normal conditions these solutions will work.

there is a bug with ubuntu that prevents web browsers from setting their font size.

with ubuntu, the font size in your web browser is set to 8 pt. and you can't change it. not with add ons, not with options.

i haven't tried zoom.

mostly because we're not in the category of visually impaired.

but if you are, this will most likely be a solution.