Two initial thoughts:

(1) Does your bank require a special plug-in or other add-on in order to sign bills? Some add-ons may be disabled during a Firefox 4 upgrade because they are not marked as compatible. These are identified by being grayed out in Tools > Add-ons. If no update is available, you might check with your bank on whether you need a new version from them or whether you might need to roll back to Firefox 3.6.

(2) Some web sites are not fully compatible with the new HTML5 parser in Firefox 4. To see whether this is the problem, you can turn it off like this:

(i) Type or paste about:config in the address bar and press Alt+Enter to open the preferences editor in a new tab. Click the button promising to be careful.

(ii) In the filter box, type or paste html5 and pause while the list is filtered

(iii) Double-click html5.parser.enable to toggle it to false (the line should turn bold).

Return to your page and reload, then try to sign again.

Does that make any difference? If so, there could be something a little bit wrong in the page, or you have found a bug in Firefox.

Hi jscher2000, Thank You for the answer. I will try what you suggested and let you know how/if it works. But it must take some time because it happen only with verification of the bills.

Hi jscher2000! I updated plug-in from the bank and all is right now! Thank You!

Hi jscher2000! It helped only for a while. When I tried to pay my bills now it happened again! So I did what you suggested with HTML5 parser despite I have FF5.0 and it didn't help either!

So please help if You can! P.S. I wrote another question becouse I couldn't find this one, sorry! https://support.mozilla.com/en-US/questions/853660

Did Firefox 5 disable your bank's add-on? You can check here:

orange Firefox button or Tools menu > Add-ons

Check both the Extensions and Plug-ins categories.

If it is disabled and there's no update available, you might have to use another browser for this site. One option is IE, of course. Another is the portable version of Firefox 3.6. I'm not sure how you use an add-on with the portable version of Firefox, though.

http://portableapps.com/apps/internet/firefox_portable/localization#legacy36

Well, I have the bank application (BankID Security Application Nexus Personal 4.18.0.10751) enabled and updated. It works great to log me in to the bank. But when I want to sign my operation (for example paying a bill) it hangs on and after a long time inform me about "technical error".

This application works perfect on IE and worked perfect on FF 4.x Isn't it a bug in FF 5.0? Isn't it a shame for FF people?

I do not want to go back to IE, what else can I do? Please, help me!

Clearly there is some difference between Firefox 4 and 5 that is affecting the BankID application, but I really have no idea what it is.

OK! Thank you anyway for trying to help me. Is there some way to send it to Mozilla as a bug in FF0.5?