Reproducable: Everytime on Windows XP SP2 fully patched & Firefox 1.0. Propably in every Windows version also. Steps to reproduce: 1. Set Firefox as your default browser if you haven't already done so 2. Create a file named "Δοκιμή Test.html" ("Δοκιμή" in Greek means Test) 3a. Make sure that Firefox is closed. 4a. Double click the file. It is opened as expected. 5a. Close Firefox But now continue from step 2. with the following steps 3b. Launch Firefox from it's shortcut 4b. With Firefox open double click the file you created in step 2. 5b. Firefox can't find the file. The reason is that the space in the filename didn't get replaced by %20 as it should. You can see that at the alert message.

Are you saying that the Greek characters are essential to reproduce the bug, and it doesn't occur with "Test Test.html"?

Yes. I have tested many times with "test test.html" and it always opens. I guess the problem would hapen with cyrillic etc charakters as well. Possible cause of the problem: "Δοκιμή Test.html" when is opened succesfully is shown in the address bar as "%C4%EF%EA%E9%EC%DE%20Test.html". Notice that greek charakters are replaced by %C4, %Ef etc and the space replaced by %20. That happens when I click on the file with firefox closed. But when firefox is open and I click the file "Δοκιμή Test.html" is replaced as I see in the alert message by "%C4%EF%EA%E9%EC%DE Test.html". Notice that the space didn't get replaced by %20 and obviously the file can't be opened. If I drag and drop the file on firefox it opens as expected. I don't know a lot about programming but it looks that when firefox converts Greek charakters to %[] it "forgets" to convert spaces to %20.

Well known bug with non-ascii chars. *** This bug has been marked as a duplicate of 263570 ***