Is Google the only major site on which you have received a certificate error?

This error code actually is not very common. Have you customized any settings related to making secure connections? You can check as follows:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful.

(2) In the search box above the list, type or paste TLS and pause while the list is filtered

If any "boolean" or "integer" values are bolded and marked as user set, you can restore their default values by right-clicking and choosing Reset. You can ignore "string" values.

(3) In the search box above the list, type or paste security.ssl and pause while the list is filtered

If any values are bolded and marked as user set, you can restore their default values by right-clicking and choosing Reset. However, you can disable (set false) these two older ciphers:

• security.ssl3.dhe_rsa_aes_128_sha
• security.ssl3.dhe_rsa_aes_256_sha

If you change anything, any difference?

Boot the computer in Windows Safe mode with network support to see if that has effect in case security software is causing problems.

• http://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/
• http://www.7tutorials.com/4-ways-boot-safe-mode-windows-10

You can remove all data stored in Firefox from a specific domain via "Forget About This Site" in the right-click context menu of an history entry ("History > Show All History" or "View > Sidebar > History").

Using "Forget About This Site" will remove all data stored in Firefox from that domain like bookmarks and history and cookies and passwords and cache and exceptions, so be cautious. If you have a password or other data from that domain that you do not want to lose then make sure to backup this data or make a note.

You can't recover from this 'forget' unless you have a backup of involved files.

If you revisit a 'forgotten' website then data from that website will be saved once again.

I resolved the problem. Some weird function on the antivirus software was resposible for the fact. Even with the antivirus stopped it was imposible to access. Now with a new virus database the problem has been resolve.

Thanks for helping.

out of interest, which av product were you using?

ESET. The Server (in charge of the updates) was runninig lower version than the clients on the PCs. Updating the clients diretly from the av company solved the problem.