When browsing with Firefox, active monitoring from antivirus products results in additional CPU usage. This active monitoring can take place in the antivirus product's own processes (e.g.
MsMpEng.exe for Microsoft Defender), but also in
firefox.exe processes themselves for antivirus products that rely on code injection. This meta bug lists work related to measuring and/or reducing the extra CPU usage that originates from antivirus products, referring to the two kinds of impact respectively as "out-of-process impact" and "in-process impact".