v35.0.1

hi KnutB, which security software is running on your system?

Bitdefender antivirus.

can you try to disable ssl scanning in bitdefender's options and see if this is making a difference? http://www.bitdefender.com/support/what-to-do-when-security-certificates-cannot-be-verified-installed-1090.html

The antivirus program is actually called "Charity Antivirus" (www.charityantivirus.com). I believe it's supposed to be just Bitdefender with a different name, but I can't find any way to configure it the way it's described in the link. I can only disable it completely, and that does not solve the problem.

unfortunately i'm not familiar with this program but if it's a derivative of bitdefender i'm positively sure that the solution will be the same as in those threads: https://support.mozilla.org/en-US/questions/1042120 https://support.mozilla.org/en-US/questions/1042111

please try to go trough all the preferences offered by the tool and see if there's a way to disable "ssl scan"...

Thank you for your patience... The solution described is for Bitdefender Total Security. Charity Antivirus is perhaps based on one of the numerous other Bitdefender products (Bitdefender Antivirus, Bitdefender Virus Scannner, Bitdefender Antivirus Plus etc.)? I could not find any way to configure it. I have now uninstalled Bitdefender/Charity Antivirus and rebooted (twice). No antivirus software on the computer. I still get the ssl_error_inappropriate_fallback_alert error.

this error is a security measure in case a web-server detects that your computer is trying to connect with ssl 3.0 (which is now considered insecure after the "POODLE" vulnerability has been discovered).

can you double check in about:config that "security.tls.version.min" is set to 1 - then firefox won't use ssl 3.0 and the issue is likely caused by some external interference.

you could also check in the firefox menu ≡ > options > advanced > network > connection - settings... if it is making a difference when you set firefox to directly connect to the internet ('no proxy').

if this doesn't work either could you go to any https://-site you can still connect to, click on the lock as security indicator on the left of the address bar and go to "more information". in this window click the "view certificate" button and see what issued by section it contains - this might give further clues if something is interfering with your secure network traffic...

I double checked the security.tls.version.min setting in about:config: 1

The options > advanced > network > connection-setting was "no proxy". I tried to change it to "auto-detect". That did not help (unsurprisingly...)

Screenshots of the security/certificate information of two functioning https://-sites here

PS: I forgot to mention (and I don't know if it matters) that I have no problem with any https://-sites in other browsers (IE and Chrome).

thanks for the detailed information - everything looks as it's supposed to be and no source for the problem is obvious.

maybe this sounds dumb, but on this similarly puzzling thread a reboot of the system helped: https://support.mozilla.org/en-US/questions/1041886#answer-678969

The reboot suggestion was definitely not dumb, because now it's working :-) Which is very strange, since I have already rebooted at least 3 times, both before and after uninstalling the Bitdefender software.

Anyway, thank you very much for your help!