Firefox suddenly displays all pages in completely unreadable font looking like a mix of Asian locales and random symbols, while language/font settings are still correct; internet page display is hence completely broken, restart/reinstall of no help.
As far as I remember I opened a Japanese web page with both EN and JP text in a new tab.
A few mn later, all my (dozens of) tabs suddenly switched to a completely unreadable scramble of symbols, a mix of Japanese and other vaguely Asian characters + apparently random symbols.
I've checked my language/font settings as thoroughly as I could by using a correctly working firefox instance on another computer and navigating through menus in parallel. No visible problem there.
When I print such an unreadable page to a file, it shows correctly in the original language (French or English). When I copy/paste some of the scrambled text, the result is also displayed fine in Latin alphabet.
Restarting firefox doesn't help, with or without saving the tabs.
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Looks that something is wrong with the font that Firefox uses.
You can try to reset the preferences:
Create a new profile as a test to check if your current profile is causing the problems.
See Basic Troubleshooting: Make a new profile
There may be extensions and plugins installed by default in a new profile, so check that in "Tools > Add-ons > Extensions & Plugins"
If that new profile works then you can transfer some files from the old profile to that new profile (be careful not to copy corrupted files)
See http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox
Thank you very much ;-) In the meantime I just restarted my Ubuntu system and everything works fine again... Very windows-like way of troubleshooting, I fear, but it's worked so far.