Try keyboard
"Ctrl" + "-" key (this is zoom out , text becomes smaller)
"Ctrl" + "+" key (this is zoom in, text becomes larger)
or try this
Start --> Run --> firefox.exe -p
if it does not work, usually these location (type exactly as follows including --> ")
"C:\Program Files\Mozilla Firefox\firefox.exe" -p
or
"C:\Program Files (x86)\Mozilla Firefox\firefox.exe" -p
Then delete the profile and then delete data
Next time you start, it should recreate it automatically.