A few days after manually - and successfully - updating Firefox to version 33, some texts on several websites suddenly disappeared, although they did work before (in the same browser version!).
Reinstalling Firefox and cleaning up my profiles didn't help. Another system (VM) with a similar outdated Linux Mint version worked flawlessy thou. Must be something else - and finally I found the cause: Apparently the font cache was in misarray.
According to a post in the LinxMint forum, you'd just have to check access permissions, set them up properly and refresh the font cache. Most times this would appear after installing massive amounts of fonts - which I didn't; but maybe some of the builds I did in-between overwrote or damaged the font cache, or maybe even overwrote default fonts .. well, happens.
So I did all what this post tells you to do, restarted Firefox - and tada, all was well again! :-)