13th March 2017, 22:00
I was afraid that might be the case. 
Ah well.
Here are three screenshots showing the last two update selections from Update Manager. I'm pretty sure none of the problems I've been having showed up before that. Yesterday was the first time I've had any trouble with my trackpad (currently, since I shut everything off last night, it's been working, hooray.), and the download-from-Firefox & laptop-not-shutting-down-when-the-lid-is-closed problems have only been in the last two weeks or so.
Thank you very much for your help.
Ah well.Here are three screenshots showing the last two update selections from Update Manager. I'm pretty sure none of the problems I've been having showed up before that. Yesterday was the first time I've had any trouble with my trackpad (currently, since I shut everything off last night, it's been working, hooray.), and the download-from-Firefox & laptop-not-shutting-down-when-the-lid-is-closed problems have only been in the last two weeks or so.
Thank you very much for your help.