How do I increase the font size in the file menu and the inbox? Turning on my PC this morning all the font-size and window-panes have changed sizes in Firefox and Thunderbird. I don't know what is going on? When I adjust Firefox it reverts back when I open a new tab or Window. In Thunderbird the font is so small in the inbox and file menu on the left side it is hard to read. But the messages appear to be unaffected.