This reminds me of users reporting that sometimes all the files want .jpe at the end instead of .jpg.

It's possible that the file which stores application handler preferences and associates file extensions with content types has become corrupted. The file has a somewhat inscrutable XML format so it's often easiest simply to hide it from Firefox and have those settings return to their defaults. Here's how you do that:

Open your current Firefox settings (AKA Firefox profile) folder using either

• "3-bar" menu button > "?" button > Troubleshooting Information
• (menu bar) Help > Troubleshooting Information
• type or paste about:support in the address bar and press Enter/Return

In the first table on the page, click the "Show Folder" / "Show in Finder" / "Open Directory" button. This should launch a new window listing various files and folders.

Leaving that window open, switch back to Firefox and Exit, either:

• "3-bar" menu button > "power" button
• (menu bar) File > Exit (or Firefox > Quit)

Pause while Firefox finishes its cleanup, then rename mimeTypes.rdf to mimeTypes.old.

Start Firefox back up again, and Firefox should rebuild the file with default settings.

Any improvement?

Followed the instructions but it didn't reset to default. Still tries to save to .jfif

Hmm, the download preferences should have been rebuilt fresh from the Windows registry.

If you have an image saved as a .jfif file, what program does Windows want to use to view/open it, if any?

The program that pops up to view the .jfif files is the photos program that comes with Windows 10

Hmm, since everyone has that program, it doesn't really explain why Firefox is behaving oddly on your system.

If you drag and drop the mimeTypes.rdf file onto a tab in Firefox, it should display with a little bit of formatting, making it slightly more readable. If you use Find (Ctrl+f) for "jfif" what content types does it show up as a file extension for? For example:

<RDF:Description RDF:about="urn:mimetype:image/jpeg" NC:fileExtensions="jpg" NC:description="JPEG Image" NC:value="image/jpeg" NC:editable="true">

I checked the document tree as you suggested. No mimetype:image listed at all. I am tempted to uninstall Firefox and then re-install it. Do you think that might help?

If there's no references to jfif in the mimeTypes.rdf file, it's very difficult to figure out where that is coming from.

If you think your Firefox program files might be corrupted, we generally suggest this approach:

Clean Reinstall

We use this name, but it's not about removing your settings, it's about making sure the program files are clean (no inconsistent or alien code files). As described below, this process does not disturb your existing settings. Do NOT uninstall Firefox, that's not needed.

It only takes a few minutes.

(A) Download a fresh installer for Firefox 48.0 from https://www.mozilla.org/firefox/all/ to a convenient location. (Scroll down to your preferred language.) For maximum plugin compatibility, choose the "Windows" version (32-bit) rather than the 64-bit version.

(B) Exit out of Firefox (if applicable).

(C) Using Windows Explorer/My Computer, rename the program folder as follows:

C:\Program Files (x86)\Mozilla Firefox

to

C:\Program Files (x86)\OldFirefox

(D) Run the installer you downloaded in step (A). It should automatically connect to your existing settings.

Note: Some plugins may exist only in that OldFirefox folder. If something essential is missing, look in these folders:

• \OldFirefox\Plugins
• \OldFirefox\browser\plugins

Any improvement?