Firefox installers only install Firefox. But I think what you are talking about are the search engines.

Firefox should not change your default search engine when you update.

However, during an update users often see a message that Firefox is performing slowly and you can speed it up by restoring some default settings. If you click that button, Firefox will perform a refresh and that definitely will reset your default search engine and unhide the other built-in search engines. The tell-tale sign is an Old Firefox Data folder on your desktop with a semi-randomly-named folder inside that has freshly updated files.

Some users unfortunately have downloaded a version of Firefox bundled with a third party "updater" that tries to push unwanted software onto your computer. If you are following update prompts from that kind of program, bad things do happen. You can double-check your McAfee scans using some of the scanning/cleaning tools listed in our support article: Troubleshoot Firefox issues caused by malware. These on-demand scanners are free and take considerable time to run. If they finish quickly and especially if they require payment, you may have a serious infection. I suggest the specialized forums listed in the article in that case.