If this is a android version you should post to the Android section of the forum here.

It is normal Firefox for Windows and it is desktop, not mobile with user agent set to Android to make instagram thinks that this is Android. Moving this thread or publish it in Android section won't give anything because it is not Android issue. Did You read my whole topic/problem or just stop only after subject?

When you upload a file, Firefox may provide the wrong content-type for it, if that was stored during a previous download. For example, sometimes sites will tell Firefox that a PDF is of content-type "application/download" instead of "application/pdf". Then if the user tells Firefox to remember that association, when they want to upload a PDF to some sites, the site will reject it because it doesn't want "application/download" type documents.

Let's see if that's a factor here. The file used to store application handling preferences changed in a recent version of Firefox, but I don't remember if it had changed by Firefox 52, so I'll ask you to look for both files.

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

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

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

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

Pause while Firefox finishes its cleanup, then:

• If you see a file named handlers.json, then right-click > Rename it to something like handlersOLD.json.
• If you see a file named mimeTypes.rdf, then right-click > Rename it to something like mimeTypesOLD.rdf.

Start Firefox back up again. Any difference?

Note that this will clear custom download handling preferences that Firefox accumulated in past browsing.

Thank You very much. Couraged by Your post I just created new profile, installed user agent switcher and it worked. Now I will try to investigate why instagram doesn't want to work on my old profile, but this is out of this problem. BTW, i don't have handlers.json.

What Firefox version are you using with the user agent switcher and did you use such an extension before with that website?

Are/were you using resist fingerprinting becaue that changes Firefox version and platform?

• privacy.resistFingerprinting = false