An add-on may be causing the problem. To test this try the procedure in the Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems article.

Safe mode makes no difference. Disabling all Plug-ins makes no difference.

As a test, try creating a new profile. User data is stored in a profile, creating a new profile will tell us if the problem i with the profile. See http://kb.mozillazine.org/Creating_a_new_Firefox_profile_on_Windows

If the new profile does work, you can transfer your bookmarks/passwords to the new profile, see http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

I recommend re-installing add-ons one at a time. It is best not to transfer the preferences, but to re-create your set-up instead.

New profile, no add-ons installed yet, makes no difference. Same problem.

Try to reinstall Firefox.

Do a clean reinstall and download a fresh Firefox copy from http://www.mozilla.com/firefox/all.html and save the file to the desktop. Uninstall your current Firefox version and remove the Firefox program folder before installing that copy of the Firefox installer. It is important to delete the Firefox program folder to remove all the files and make sure that there are no problems with files that were leftover after uninstalling.

See http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Clean_reinstall

I've done this again, for the fourth time, and the problem persists.

Uninstalled 3.6.8 including full registry cleaning (Ccleaner). Installed 3.5.3 (sunday august 8): problem over. Received update 3.5.11 (monday august 9): Problem is there again. Resetting PC with systemreset (three months back) made no difference. I have now, again, installed 3.5.3, and disabled updates.

> I have now, again, installed 3.5.3, and disabled updates.

I think it bears saying that you are operating a version with dozens of known security vulnerabilities in order to regain one time-saving feature. Perhaps you should consider other solutions.

Does this problem arise only on particular sites or very slow loading ones? I can't readily replicate it.

I'm open to any solution you can offer, but I have no other option till the problem is solved. The problem arises on every website, no matter how slow or fast it is. Further, I don't think you might be able to replicate it unless you have the same problem.

You did delete the Firefox program folder before reinstalling Firefox ?
Doing that is important to make sure that all files are removed.

Does it still happen with all plugins disabled?

Do a malware check with a few malware scan programs.
You need to use all programs because each detects different malware.
Make sure that you update each program to get the latest version of the database.

• http://www.malwarebytes.org/mbam.php - Malwarebytes' Anti-Malware
• http://www.superantispyware.com/ - SuperAntispyware
• http://www.safer-networking.org/en/index.html - Spybot Search & Destroy
• http://www.lavasoft.com/products/ad_aware_free.php - Ad-Aware Free
• http://www.microsoft.com/windows/products/winfamily/defender/default.mspx - Windows Defender: Home Page

See also "Spyware on Windows": http://kb.mozillazine.org/Popups_not_blocked and What to do when searches take you to the wrong search website

I did remove all Mozilla folders and files after removal and before reinstalling. I removed all registryentries using Ccleaner. Disabling all plugins and add-ons made no difference. Startup in Safe mode made no difference (same problem). Adding another profile made no difference (same problem).

I use the following programs: AVG, automatically. Avast, manually once a month Windows defender, automatically Malwarebytes, full scan manually once a month and recently as related to the problem. No malware/spyware found. Spybot S&D, same. All these programs are up to date.

Is it possible that any of your real-time security products intercepts web requests and investigates the URLs before returning the pages? I still think Firefox should behave normally, but as you point out, I can't exactly replicate your configuration.

Random further thought: once a page has partially loaded, the browser is aware of its URL. You could save this bookmarklet to display the full URL and navigate to an edited version. It's not very nice UI, but maybe it will help?

javascript:var sUrl=prompt("Go to new URL on "+location.host,location.href); if(sUrl!=null) location.href=sUrl; void 0;

(Copy the above string of code. If you use the Bookmarks toolbar, right-click it, choose New Bookmark, and paste the code into Location, then give it a useful name. Then you can click the button to run it when needed.)

When the problem occurred the first time, I tried this by disabling AVG and Windows Defender. This made no difference. The strange thing in this whole situation is that, now I use Firefox 3.5.3, the problem does not occur and Firefox behaves as it should be.