Use Revo Uninstaller Portable in Advanced Mode to remove the 32-bit version and then proceed to install the 64-bit version.

Thank you, I know Revo but I think it is the last solution. I could also delete most of the Firefox files and than clean the system but I want to understand, what happens here. I tried as admin and in safe mode but helper.exe doesn't start. I tried ProcessExplorer to see if an other program locks helper.exe, but nothing. Any other ideas welcome!

You can try using Unlocker to identify what process is blocking that file, or running the process in Safe Mode.

Unlocker didn't find any locking program. I followed an other advice here, renamed Mozilla Firefox in (x86) as .old (as admin) and reinstalled Firefox 32bit. Than I could delete it and install the 64bit version.

I assume it had something to do with "Firefox for Acer"...

It may be. You always want to download Firefox from Mozilla official page only.

Other than Acer, you could sometimes be up with Trojans.

You are absolutly right. This 32bit version was preinstalled I guess by Acer at a new laptop. I cann't understand why they not install the 64bit version on a brandnew machine...

Thank you

It's a matter of what kind of system images they use and maybe of what agreements they might have with other companies for them to come up with bundled software.