Try to delete search.sqlite and search.json in the Firefox Profile Folder.

• http://kb.mozillazine.org/Profile_folder_-_Firefox

Also make sure that cleanup software isn't corrupting files in the Firefox Profile Folder.
If you use virtualization software or your security software offers that feature then try to turn that feature off as a test.

Which security software (firewall, anti-virus) do you have?

my security software is avast, dutch free software. but pretty good.

See http://www.avast.com/pr-avast-6-0-will-bring-automatic-virtualization-technology-to-all-free-users

deleting those files, doesnt solve the problem. when starting firefox, search engines are still gone...

Are only default search engines missing or can't you install any new search engines?

Do you see the default search engines in the search engines in the searchplugins folder in the Firefox program folder (C:\Program Files\Mozilla Firefox\) ?

Do a clean reinstall.

Download a fresh Firefox copy and save the file to the desktop.

• Firefox 7.0.x: http://www.mozilla.com/en-US/firefox/all.html

• Uninstall your current Firefox version.
• Do not remove personal data when you uninstall the current version or you lose your bookmarks and other data in the profile folder.

Remove the Firefox program folder before installing that newly downloaded 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.

Your bookmarks and other profile data are stored elsewhere in the Firefox Profile Folder and won't be affected by a reinstall, but make sure that you do not select to remove personal data if you uninstall Firefox.

• http://kb.mozillazine.org/Profile_folder_-_Firefox
• http://kb.mozillazine.org/Profile_backup

after clean reinstall problem still occurs. In my seach plugins folder there is only one search engine, that from google. But also this one, doesnt work.

See:

• http://kb.mozillazine.org/Unable_to_search_or_add_new_engines

problem still occurs, and I also shut down the auto sandbox function on my virus software.

Did you remove the Firefox program folder with all files and folders in it?

You can try to reboot the computer after having done that to verify that that folder is still missing and hasn't been restored.
Those default search engines should be there unless other (security or malware) software has removed them

Do a malware check with some malware scanning programs.
You need to scan with all programs because each program detects different malware.
Make sure that you update each program to get the latest version of their databases before doing a scan.

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

See also:

• "Spyware on Windows": http://kb.mozillazine.org/Popups_not_blocked

tried a couple of scanning programs, much of them didnt detect anything, one did, couple of tracking cookies, which I deleted, but problem still occurs.

I wonder why you have only one default search engine in your default plugins folder when I have six of them in my Dutch Firefox: bing.xml, bolcom-nl.xml, google.xml, marktplaats-nl.xml, wikipedia-nl.xml and yahoo-nl.xml. These six names also show up in the search.json file. Did you completely turn off your security software while installing? It's also possible that you have some kind of toolbar messing with your setting. Personally I do not like toolbars because they can create all kinds of strange problems.

we kunnen ook gewoon nederlands praten ;) maar iedere keer na een herinstallatie verdwijnen die searchengines. en heb ook al een aantal malware scan programma's gebruikt, zoals hierboven aangeraden. Maar de fout blijft zich voor doen. Voor dat ik de computer afsluit, werkt het, start ik de computer opnieuw op, zijn ze weg.

Worden er bestanden gewijzigd of verwijderd tijdens het opnieuw opstarten en zo ja, welke dan? Dit kun je vaststellen door een kopie te maken van alle bestanden die zijn betrokken bij de zoekmachines (de xml bestanden, de search.json en de search.sqlite) voor het herstarten. Vervolgens doe je hetzelfde na het herstarten. En graag ook na de herinstallatie goed controleren of alle bestanden aanwezig zijn. Ook kun je controleren of er iets veranderd wordt in de instellingen bij about:config. Voor en na het opnieuw opstarten moeten de instellingen precies hetzelfde zijn.

If files and folders get lost after a reboot then you have some virtualization or cleanup software running that is restoring files to their previous state like I wrote above. Files do not get removed without a reason.

Are the files still there if you boot the computer in Safe mode via F8 on the boot screen?

after uninstalling my anti virus software, en reinstalling firefox, problem still occurs, after restart. The only searchengine that stays in the folder is google.xml, and even that searchengine doesnt work. And I cant reset firefox to the default searchengines... so I really dont get what the problem is.