This was driving me nuts. I reinstalled, cleared cache, reset Firefox (hassle to come back from), tried Safe Mode, and about 15 other things. None fixed it.
Answer is arcane, but easy.
Problem is you have Kaspersky anti-virus. It has a bug (or “feature”) where if you don’t have their browser extension, you get problems like this, and often, specifically this maddening problem.
To fix it you have to uncheck “Inject script into web traffic to interact with web pages” in the network settings.
(which I don’t recommend adding the browser extension, This is an ABSURD amount of permissions to give anything. Some of it makes sense, some of it a smart person wouldn’t give their best friend:
Access your data for all websites
Access your data for sites in the “named” domain
Access your data in # other domains
Access your data for a specific site
Access your data on # other sites
Read and modify bookmarks
Read and modify browser settings
Clear recent browsing history, cookies, and related data
Get data from the clipboard
Input data to the clipboard
Extend developer tools to access your data in open tabs
Download files and read and modify the browser’s download history
Open files downloaded to your computer
Read the text of all open tabs
Access your location
Access browsing history
Monitor extension usage and manage themes
Exchange messages with programs other than Firefox
Display notifications to you
Provide cryptographic authentication services
Read and modify privacy settings
Control browser proxy settings
Access recently closed tabs
Access browser tabs
Store unlimited amount of client-side data
Access browser activity during navigation
Access MIDI devices with SysEx support