What file did you download?

If you get a pop-up message asking to update Firefox or plugins or scanning for malware then such a message is likely a scam and you should never respond to such an alert to avoid getting infected with malware.

• Only update Firefox via "Help > About" or by downloading and installing Firefox from the Mozilla server and never via a pop-up or link on a web page.
• plugins should only be updated via the plugin itself or by visiting the home page of the plugin.

See also:

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

No it was not a malware download, I was downloading a backup from my server. I just mentioned it because it was a relatively large download with 600MB.

In any case, after restarting my computer, Firefox has resumed working again without problems.