Try updating to FF54 current build and see if fixes problem. Also since know problem, fix is to stick Sys internals back in which is not really a big issue if that's all the problems you have.

Note that the OP is on the Windows XP platform and thus has been moved to the Firefox 52 ESR version.

• https://support.mozilla.org/en-US/kb/end-support-windows-xp-and-vista

I don't think that Firefox or its updater would be capable of removing other applications, so something would be happening.