Let me provide some additional info on the problem: It has been a nice trick for years to easily create *.xls files via PHP by just creating an HTML table, and delivering it with an MS Excel mime type and an xls suffix. Browsers then saved the file to disk, or had Excel open it, which works well.

I can't say when this stopped working in Firefox. My current configuration is: Windows 7, Firefox 9.0.1, MS Office Starter. When I try to download this type of XLS file, Firefox seems to analyze the content and add an .htm extension to the filename, regardless of what is set in the application settings.

This may be a feature rather than a bug - however, it breaks a lot of existing web applications. I can locate the downloaded file in the temp folder, remove the .htm extension and open the file with Excel. But this is not acceptable for my customers; I'd rather give them the advice to use IE for applications that create XLS files.

Update: I just noticed that the above happens only when I choose "Open with" in the download dialog. It happens though the MS Client Virtualization Handler is correctly set as the application to open the file.

OTOH, when I choose "download to disk", the file extension will not be changed, and I can open the file with Excel by normally doubleclicking it.