When you install Firefox distributed by a third party, the third party sometimes modifies the bundle. You may find that it's normal for your distribution to have that extension (as a Windows user, I wouldn't know). You certainly could disable it if you like.

Regarding the script error, did this article pop up during your question submission: Warning Unresponsive script - What it means and how to fix it. I'm not sure about the malware warning, that may be coming from a different program.

No, I was resetting to defaults because when searching for info on the unresponsive script I was told that unidentified browser add-ons were likely sources of this malware. (secure-ds.serving-sys.c ... .js) When UBUNTU modifications 3.2 wasn't disabled with the rest of the add-ons, it made me suspicious. I don't know yet if the problem has returned.