@bastyani
Try renaming it to .doc or .xls Use the terminal just to copy file with same name and different extensions. Also verify via terminal the file is there. You must see it in Documents app before you can select it in FF plugin.

Terminal shows that the file is really there, but still other apps don't see it. Any other way to select the file for the FF plugin? Something like "Astro" that I used on my Android phone to select files?

Can you try just placing normal .txt there to see if it seen by the Documents App?

Ofcourse you are aware that with incepted device you can install and remove any pem CA's to your device with acmcli command? So you can have your selfsigned CA as trusted etc.

As a hint "acmcli -lc common-ca -L" gives you list of currently installed SSL CA's.

Disclaimer: if you don't know what your doing, then don't touch acmcli command. You have been warned!!!

I even tried to attach a .txt file and sent it to my mail and then downloaded the attachment. The attachment is clearly visible in the Documents folder (in Filebox) but not visible in the Documents app.
As far as the "acmcli" command I really don't know how to use it but as I said I also used the system way of importing my certificate (Settins-Security-Certificates) and it shows that the certificate is imported, but it does not work. I am not sure if Firefox can use the certificates imported in the system or they have to be imported in Firefox as well?

@bastyani
FF can NOT use certificates imported in the system. You seem to have some issue with your tracker updating. That is why you don't see new txt files in your Documents folder. If you resolve issue of document files not displayed in Document app, than you will be able without any problems to import certificate to FF and use it.
Try rebooting your phone.

PS. Do you have issues with files when populating Gallery app?
If you move image to /home/user/MyDocs/DCIM ot Pictures - does it show in Gallery app?

Hi guys! Sorry for this delay, I was very busy at work.
So somehow I managed to install my certificate in Firefox. I guess it was a system problem as my files didn't refresh and I was unable to see my files. Then after a reboot and some gymnastics I successfully imported the certificate in FF. Big thanks to everyone, especially to Zlatko for the help. Unfortunately now there's a second problem - in the moment I click on my online bank link (which is https://...) Firefox immediately crashes. Any ideas why FF crashes?

@bastyani
Happens to me from time to time as well. But after several attempts it starts to connect OK. Just try several times.

Tried many many times, rebooted phone, FF still crashes each time...
Tried both the classic and mobile version of the bank site.
Is there some kind of error/crash log that FF has to check out the problem?

@bastyani
I have tried another cert - got same behavior as you. So it works with certain certs and does not with other. You can try write e-mail to FF support, but I doubt anybody there will care about N9's version.
Lets hope built-in browser security features will be improved in PR1.3.