firefox increase menu font size, bigger font

1. In a new tab, type or paste about:config in the address bar and press Enter. Click the button accepting the risk.
2. In the filter box, type or paste devp and pause while the list is filtered
3. Double-click layout.css.devPixelsPerPx and change its value to 1.5 and click OK. That corresponds to 150% of the classic font size.

Too big? Try 1.25
Too small? Try 2.0
Don’t choose a value below 1.0 or about 4.0 or you may find it difficult to get back to a viewable Firefox! If something does go terribly wrong, right-click the preference and then tap the R key to reset to the starting value.
https://support.mozilla.org/en-US/questions/1239467