If you are upgrading to Firefox 18, 19 or newer on a slightly older system (say Ubuntu 10 LTS), you will see the following error message :
libxul.so: undefined symbol: _ZNSbItN4base20string16_char_traitsESaItEE4_Rep20_S_empty_rep_storageE
The solution is to use Firefox 17.0ESR. It's a better version anyway.
Ideally you will also make your displeasure known to the idiots at Mozilla