You are not infected if this so called virus was only found in the Cache folder as it is harmless there if left there. It will either get overwritten over time as Cache gets used or deleted if you clear the Cache. Also you are using macOS and not Windows ;)

Old article with older Firefox settings but it is still applicable. https://support.mozilla.org/en-US/kb/Firefox%20cache%20file%20was%20infected%20with%20a%20virus

If you want to clear your Cache: Settings -> Privacy & Security Cookies and Site Data -> Clear Data -> [X] Cached Web Content -> Clear https://support.mozilla.org/en-US/kb/how-clear-firefox-cache

If you get a pop-up on a webpage then simply close this tab via the close X on the Tab bar, do not click in such a pop-up.

You can do a malware scan, if you aren't sure.

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware