I just finished installing Mint Linux in my machine (Cinnamon Version 64 bits). I opened Firefox browser and the fonts were so small to read a Web Site.
I started my research about how to fix this, as i don't want to change the Zoom in/out configuration in Firefox continuously.
After a few minutes i was able to fix it by doing this:
Firefox has an internal setting that governs both the “chrome” area (toolbars, menus, dialog) and the content area (pages). If you want to change this, you might need to adjust the settings:
(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful.
(2) In the filter box, type or paste pix and pause while the list is filtered
(3) Double-click layout.css.devPixelsPerPx and change its value to 1.25 for 25% enlargement from the classic default of 96 pixels per inch, and click OK to see whether that’s any better. I did it with 1.5 to have a 50% enlargement,
Try what suits your preferences.
Hope it helps.