Here are some troubleshooting steps you can try:

1)Clear your Firefox cookies and cache: Go to the Firefox menu, click on "Options," then "Privacy & Security," and under "Cookies and Site Data," click "Clear Data." Make sure to check the boxes next to "Cookies and Site Data" and "Cached Web Content," then click "Clear." 2)Disable Firefox extensions: Some Firefox extensions can interfere with websites, including Facebook. Try disabling all of your Firefox extensions to see if that resolves the issue. Go to the Firefox menu, click on "Add-ons," then "Extensions," and disable all extensions. 3)Use Facebook in a private browsing window: Open a private browsing window in Firefox and try logging into Facebook there. If it works, then the issue could be related to your Firefox cookies and cache, so you may need to clear them.