Set:

• Windows 10
• Firefox for Windows
• A stable internet connection

Possible Causes:

• A Firefox update gone bad
• Cached data from (some) sites may be incompatible after an Firefox update

Solutions:

If Firefox is still somewhat responsive:

Delete the cached data in Firefox for the site or all sites

• In Firefox go to "Settings" - "Security" - "Cookies and Site Data" - "Clear Data"
• Clear all or some site specific data.
• Close Firefox then try again to open the site.

If Firefox is frozen for good:

Create a new user profile

• Press "Windows Key" and "R - Key" simultaneously.
• A little Window should pop up.
• In this window please write: "firefox.exe -p"
• Press "OK" or "Enter"
• Choose "Create new profile"
• Choose your new profile as standard.
• Note down the name of your old Firefox profile
• Start Firefox with the new profile
• In Firefox go to "Settings" - "Manage Bookmarks" - "Manage all Bookmarks"
• Then choose to "import bookmarks"
•  Search for the old firefox profile
• Import the bookmarks from you old profile