Firefox doesn't open dynamical html-files anymore: "unnamed.html could not be opened"

Hi all,

I got a problem that is probably related to the firefox U3 application, but possibly also a general firefox problem. But I suspect the origin lies in the U3 version.

I'm using Firefox U3 portable (version 1.5.0.8 ) for some time now, at home and in the office, without much trouble. But 2 weeks ago a pecular problem started: Whenever a html link leads to a html file via a dynamic link, like a php script that's returning html content, firefox pops up a dialog box whether to store the file or open it using an executable (which is normal behaviour). Usually I just open it using the default executable, which is firefox. Internally, the html file is stored to a file "unnamed.html" in the temp directory. But since 2 weeks this mechanism doesn't work properly anymore, I get a dialog box "...\unnamed.html could not be opened, because an unknown error occured. Try saving to disk first and then opening the file." afterwards. The file unnamed.html was created though, and I can drag it to the browser (that is, before I close the warning dialog, because it will be removed automatically otherwise). I also run sysinternals filemon in parallel and observed that all files were created (first a randomized xyz.html that was then renamed to unnamed.html), but this file was never actually read by any program. Note that this doesn't happen with normal html links, only with calls to server-side scripts that happen to return html files (often the case with file attachments to mails on hotmail and similar).

In a second step I browsed to a text editor in the first dialog box, using its "Browse" button. This worked perfectly. Finally I used the "Browse" button to chose the firefox.exe executable of my U3 stick (N:\System\Apps\3C9F7B3F-D55C-42cd-8537-B878518B73AF\Exec\firefox\firefox.exe) and checked the box "Do this automatically for this type of file from now on", and now it is working again perfectly as before. So this is a workaround for the problem, but the cause for the problem is still unknown. Did anybody else observe something similar?

I suspect firefox doesn't succeed to find "itself" (or its own executable) for opening the html file for some reason, maybe due to a wrong pathname or changed drive letter? Funnily I observe the same problem with the local installed normal firefox application (non-U3), but the problem occured first with the U3 application (and I didn't use the local anymore since I got the U3 version, just after the error occured for verification).

[EDIT] It seems the problem is that, for some unknown reason, firefox was no longer my default browser; double-clicking a html file didn't open the file automatically. By setting the default browser back to firefox, it is working fine again now. The question remains why my system(s) suddenly didn't have *any* default browser set anymore (can't be IE, as this would change the default browser in the worst case, but not clear this setting...)