I think you can change that see the Use other... from the drop-down menu in this link : Options window - Applications panel

see also : Managing file types

thank you

Please mark "Solved" the answer that really solve the problem, to help others with a similar problem.

ideato,

As I mentioned in my original post, I already have the options set for MIME types image/tiff and image/x-tiff to use AlternaTIFF.

See if you can disable TIFF in the QuickTime settings.

I already used the Quicktime control panel to uncheck both TIFF entries in the browser tab, MIME types section.

So, you no longer see TIFF under one of the QuickTime plugins on the about:plugins page?

In that case you can try to delete the mimeTypes.rdf file in the Firefox Profile Folder in changing the setting in "Tools > Options > Applications" doesn't work.

• http://kb.mozillazine.org/mimeTypes.rdf
• http://kb.mozillazine.org/File_types_and_download_actions#Resetting_download_actions

You can use this button to go to the Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Open Containing Folder

Now we are getting somewhere...

In examining about:plugins, I see that Quicktime 7.7.1 has seven sections! One for each of npqtplugin7.dll, npqtplugin6.dll, npqtplugin5.dll, npqtplugin4.dll, npqtplugin3.dll, npqtplugin2.dll, npqtplugin.dll.

Only the section corresponding to npqtplugin7.dll contain and image/tiff reference. I guess this explains why Firefox is using Quicktime instead of AlternaTIFF despite its options being set otherwise.

I tried deleting mimetypes.rdf, no improvement.

In looking at about:config, there are no entries for

browser.download.pluginOverrideTypes

or

plugin.disable_full_page_plugin_for_types

You can try to remove (delete) that interfering QT plugin if you do not need any of the other MIME type supported by npqtplugin7.dll
QT can be very persistent with wanting to handle media files.

See also:

• http://kb.mozillazine.org/Issues_related_to_plugins#Identifying_installed_plugins

I guess.

LOL, I don't know how Firefox is architected, but if a plugin can be "persistent" then that's definitely a problem. Seems to me Firefox should have the last word.

QT plugins can be persistent because QT restores them if you make changes to the QT MIME type settings after you've removed a plugin manually by deleting the plugin file.

OK. Will a future version of Firefox be able to fix this or is it Apple's issue?