We are serving files with the .xlsx (excel 2007/2010) extension. IE, Chrome, Safari all download the file and open excel just fine.

For some reason Firefox is appending .xls to the extension. I found this: https://support.mozilla.com/bs/questions/758363

However, the instructions weren't very specific and completely unclear to me.

How can I tell Firefox not to screw with the file extension?

I'm using 3.6.13 on Windows 7.


  • Resetting automatic actions, as indicated in the link you provided, is per client, so you'd have to change the setting on each install of Firefox.
    – user3463
    Dec 30, 2010 at 2:14
  • @Randolph: Per client is okay as I'm just trying to fix two machines. I went through the instructions but that didn't work. Either the instructions are bad or I completely misunderstood what to do.
    – NotMe
    Dec 30, 2010 at 15:20
  • it's entirely possible that your MIME types are messed up, especially with Firefox. I really don't know what to suggest if that page didn't help (though to be fair, I only read through it and didn't actually try anything).
    – user3463
    Dec 30, 2010 at 20:51

1 Answer 1


Instead of changing Firefox, I found that the site in question was sending the wrong mime type for office 2007/2010 documents. Fortunately, they were kind enough to fix it.

I think Firefox ought to leave the extension alone, but I doubt that's going to change anytime soon given the severe lack of attention to the many bugs filed about it.

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