You may have accidentally zoomed webpage(s). Reset the page zoom on pages that cause problems.

• View -> Zoom -> Reset (Ctrl+0/Command+0 (zero))

• https://support.mozilla.org/en-US/kb/font-size-and-zoom-increase-size-of-web-pages

If you have made changes to Advanced font-size settings like increasing the minimum/default font-size then try the Default font-size setting "16" and the Minimum font-size setting "none" in case the current setting is causing the problem.

• Settings -> General: Fonts & Colors -> Advanced -> Minimum Font Size (none)

Make sure you allow pages to choose their own fonts.

• Settings -> General: Fonts & Colors -> Advanced
  [X] "Allow pages to choose their own fonts, instead of your selections above"

It is better not to increase the minimum font-size, but to use the built-in Firefox Zoom feature.

• Settings -> General -> Language and Appearance -> Zoom

Many thanks cor-el. Trying various combinations of View > Zoom and Settings fixed it - though there is no "View>Zoom>Reset" in my Firefox 98.0.1. I suspect I caused the problem trying to do superscript/subscript in WORD with CTRL + + when in the wrong window!