Try temporarily disabling any third-party antivirus or security software.

Visit about:third-party in the address bar and try disabling third-party modules.

You may also want to check for any Enterprise Policies that could affect your ability to install add-ons. To view active policies:

Type or paste about:policies in the address bar and press Enter to load the page.

Is there anything on the Active list? If not, it will say "The Enterprise Policies service is inactive."

The most common source of Enterprise Policies is an IT department on a work PC. But they sometimes are added by other sources...

Zeroknight, I blocked all third-party modules, restarted, and still could not install the Bitwarden extension. Three of those third-party modules were from Trend Micro anti-virus. This is a work computer and I am unable to disable the desktop Trend Micro client. Is it possible that the desktop program is the cause, even if the modules are disabled in Firefox?

jscher2000, thanks for the tip. I looked and there were no enterprise policies active.