It appears to be an issue on Roblox's end. It needs to be fixed by their developers.

The issue with Roblox should be fixed by clearing the cookies and site data associated with Roblox.

The easiest way to do this, without impacting other websites, is to go to the Roblox website and click the lock icon that appears in the left side of the Firefox address bar. From the popup that appears, click Clear cookies and site data.... The next popup that appears may take a little while to appear.

When it appears, it will show you a list of the websites that are being cleared and you can press the Remove button. If the website doesn't reload on it's own, press the refresh button (the swirl arrow on the Firefox toolbar or F5 on your keyboard).

After you log back in, that should help you.

After looking into the issue further, it looks like this is a bug that impacts more than just Roblox. A workaround to solve the problem (if clearing the cookies doesn't fix the issue or if the issue happens repeatedly) is to do the following:

1. Type about:config into the Firefox address bar and hit enter.
2. Bypass the warning message (if there is one).
3. In the search bar, find the security.allow_disjointed_external_uri_loads preference.
4. Press the toggle button on the right side of the preference or double click the preference value to change it to true.
5. Restart Firefox.