Do you use any custom themes in Firefox? Or, any Add-ins?
If so, you might try opening Firefox in "safe mode" (with the default theme and disabling all add-ins) to see if they are affecting it. I use Firefox consistently all the time on Ancestry with no issues. I have had other sites go wonky with formatting when I used the +/- settings in a way they didn't expect.
There is an article here for putting Firefox into "safe mode" http://support.mozilla.org/en-US/kb/troubleshoot-firefox-iss...