It looks that you got some HTML error page and not JSON data.

You can try these steps in case of issues with web pages:

You can reload web page(s) and bypass the cache to refresh possibly outdated or corrupted files.

• hold down the Shift key and left-click the Reload button
• press "Ctrl + F5" or press "Ctrl + Shift + R" (Windows,Linux)
• press "Command + Shift + R" (Mac)

Clear the Cache and remove the Cookies for websites that cause problems via the "3-bar" Firefox menu button (Options/Preferences).

"Remove the Cookies" for websites that cause problems:

• Options/Preferences -> Privacy & Security
  Cookies and Site Data: "Manage Data"

"Clear the Cache":

• Options/Preferences -> Privacy & Security
  Cookies and Site Data -> Clear Data -> Cached Web Content: Clear

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

You can remove all data stored in Firefox for a specific domain via "Forget About This Site" in the right-click context menu of an history entry ("History -> Show All History" or "View -> Sidebar -> History").

Using "Forget About This Site" will remove all data stored in Firefox for this domain like history and cookies and passwords and exceptions and cache, so be cautious. If you have a password or other data for that domain that you do not want to lose then make sure to backup this data or make a note.

You can't recover from this 'forget' unless you have a backup of involved files.

If you revisit a 'forgotten' website then data for that website will be saved once again.

• https://support.mozilla.org/en-US/kb/delete-browsing-search-download-history-firefox

The solution is not a solution; in fact all you need to do is hit Ctrl-F5 and the page will load correctly.

However, it'll only work for a while (a day or a week, not sure) - after a certain time (or if you empty your cache) the error will pop up again; a json error on a HTML page.

For some reason it seems Firefox thinks it has encountered a json file. I cannot fathom why. It maybe caused by some add-on, dunno.

How sad, from time to time I triy to go back to Firefox as my primary browser, but something always gets in the way, in this case I'm hitting the issue described here. Having to go over all these cache clearing steps (especially for established, arguably "correct" sites out there --AWS, xfinity, ... -- is a real hassle. I've never seen a misbehavior like this in other browsers.

Hey guys, I too had this problem, just solved it. A plugin named JSONView was causing this issue for me with AWS Console. You can start firefox in safe mode and check if this issue is caused by a plugin and then remove or disable them.