Perhaps, I hadn't thought that any significant security issues had been found in recent versions. Are there any I should be concerned about? If not, my reasoning is that I don't want to break what is working perfectly fine for me as is.

After looking at https://www.mozilla.org/security/known-vulnerabilities/firef... there doesn't seem that there are any to be concerned about. However, in case you are right, I'm recompiling a more recent version of Firefox.