I found this suggestion in another thread:

Enter about:config in the address bar and press Enter Click on "Accept the Risk and Continue" In the search bar enter layout.css.devPixelsPerPx Click the Edit button at the right In the edit window set it's value to 2 and press Enter. (1.3 or 1.4 works better for me) When you're finished, close the about:config tab.

As far as colors are concerned, maybe this article can help: Change the fonts and colors websites use

Also, I hope that you're aware of the global Zoom at: 3-bar Menu button -> Options -> General page (at the left) scroll down to Zoom.

and Firefox Reader View for clutter-free web pages

Browse Safe & Keep Healthy !