Hmmmm.... I don’t know if this will help you, but
You can ZOOM IN/OUT from your browser by pressing CTRL and + (to enlarge or zoom in) or CTRL and - to zoom out(make smaller). CTRL and 0 will reset to NORMAL.
Also you say you use 150% font, which means you mean the system font settings. Browser settings are usually done by setting FONT SIZE. They are two different things.
Try setting your system fonts back to a smaller percentage , see if that corrects things.
Then try changing the font size in your browser (tools/options/content).
If all else fails, do the CTRL+ or CTRL- to adjust things to make it readable.
Hope this helps.
P.S. To repeat, Windows font size settings and Firefox Browser font size settings are two different things.
Windows font size is usually done through the CONTROL PANEL/DISPLAY/APPEARANCE settings (on an XP).
Windows 7 may be different.
P.P.P.S In my first post where I used the words “system font”, I meant “windows font”.