I switched from Firefox to Chrome right after I discovered that malware on Firefox had changed so many about:config settings that I couldn't untangle my own changes to about:config from the changed made by the malware.

That and my having seen repeated assertion by HN user tptacek that Chrome's security team is the second best in the industry (behind only the iOS security team).

Well, that and the lack of any signs that Mozilla management was prioritizing improving Firefox's resistance to malware. (Mozilla's interest in using Rust I saw as a positive sign, but not particularly directly relevant to the question.) At the time, one of Mozilla's priorities was introducing a new mobile OS.