Open Options > Content: Under fonts&colors you change change the text size. See if that is what you are looking for.

Hi, I think you may be referring to zoom, so please read [https://support.mozilla.org/en-US/kb/font-size-and-zoom-increase-size-of-web-pages?redirectlocale=en-US&redirectslug=how-do-i-use-zoom this article] - note, you can also zoom by holding down Ctrl and turning the mouse wheel. ''If you receive a satisfactory answer, please help others having a similar problem, by marking it as 'Solved.'''