Maybe the default zoom level has been changed. In that case, to reset size back to normal, press and hold Ctrl while pressing 0 (zero).

If that doesn't help, please try resetting Firefox. It will save all your bookmarks, passwords, etc, but get rid of any non-default settings and add-ons. Here is how to reset Firefox: Refresh Firefox - reset add-ons and settings

Can you attach a screenshot?

• http://en.wikipedia.org/wiki/Screenshot
• https://support.mozilla.org/kb/how-do-i-create-screenshot-my-problem

Use a compressed image type like PNG or JPG to save the screenshot.

Start Firefox in Safe Mode to check if one of the extensions or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance/Themes).

• Don't click the Reset button on the Safe mode start window or otherwise make changes.
• https://support.mozilla.org/kb/Safe+Mode

You can check for problems with the preferences and if necessary delete the prefs.js file and a possible user.js file to reset all prefs.

• http://kb.mozillazine.org/Preferences_not_saved
• https://support.mozilla.org/kb/Preferences+are+not+saved