-> go to View Menu -> Zoom -> click "Reset"

-> go to View Menu -> Page Style -> select "Basic Page Style"

-> go to Help Menu -> select "Restart with Add-ons Disabled"

Firefox will close then it will open up with just basic Firefox. You can enable the Add-ons later. Check and tell if its working.

If you have increased the minimum font size then try the default setting "none" as a high value can cause issues like you described.

• Tools > Options > Content : Fonts & Colors > Advanced > Minimum Font Size (none)
• Tools > Options > Content : Fonts & Colors > Advanced > [X] "Allow pages to choose their own fonts, instead of my selections above"

You can use one of these extensions to set a default font size and page zoom on web pages:

• Default FullZoom Level: https://addons.mozilla.org/firefox/addon/default-fullzoom-level/
• NoSquint: https://addons.mozilla.org/firefox/addon/nosquint/