There are some bug reports about that, like bug 57342, which mentions this extension: Open in Browser.

Thanks for the quick reply, gonna see if that addon can fix my problem.

works wonder! check out this example: http://wallpaperswide.com/download/bike_chase-wallpaper-2560x1600.jpg firefox claims it is an WinRAR archive but it's clearly an image

That file is send with these headers:

Content-Type: application/force-download
Content-Disposition: attachment; filename=bike_chase-wallpaper-2560x1600.jpg

Code like this will also make Firefox display the image without the need of an extension.

data:text/html,<img src='http://wallpaperswide.com/download/bike_chase-wallpaper-2560x1600.jpg'>

“WinRAR archive” probably depends on what you’ve downloaded before; Beta with my profile says it’s a Gzip archive, Firefox with a new profile says it’s JPG.

Sounds that you have previously set an automatic download action for the application/force-download MIME type.

• http://kb.mozillazine.org/File_types_and_download_actions
• https://support.mozilla.org/kb/Managing+file+types

I backed up my mimetypes.rdf and removed all entrys with force-download and restarted firefox, but the only difference it made is now firefox says it is an jpg but it still wants to download it/open in external program. If I go to Options->Applications the jpg entry is there but I can't select preview in firefox.

Also found this "Is Do this automatically for files like this from now on disabled? This can happen if the website's server incorrectly specifies the MIME type of the file. It also can happen if the server assigns "Content-Disposition: attachment" to the file."

Is it possible to make some automatic action if(jpg and Content-Disposition: attachment)

     show in firefox

or is it some brute force way to add "show in firefox" in the options->application for a certain filetype/mimetype

So I was thinking if I could do a greasemonkey script that dynamically modified the response header into a image/jpeg so I just add each webpage that had this problem in the include, but gave up quite fast.

So for now, ill use Open in browser together with this autohotkey script (may need some tweeking)

#IfWinActive Opening ahk_class MozillaDialogClass
i::Send !p{tab 2}i{enter}
#IfWinActive

(press i to choose View as Image)