I sometimes show my age by making jokes about Bonsai Buddy and Gator and Hotbar, but ads injected in browsers are a problem that’s coming back. And sometimes these ads come with malicious payloads, installing unwelcome software on your computer to maintain persistence.
Problems like this are the reason I tend not to load my browsers down with lots of extensions. Sometimes the functionality is cool, but I’ve always found ways to get what I need done with a stock browser, and then I have a better idea of what I’ve gotten myself into. I’m beholden enough to the agendas of Microsoft, Mozilla, or Google as it is; I don’t need third parties injecting their agendas into the mix, especially when they may be malicious.
And besides that, a lot of extensions tend to be very memory- or CPU-hungry. I have enough memory on most of my machines that I can dedicate 2 GB of RAM to a web browser, but I’m not sure why I should have to.
The fewer extensions you load onto your web browsers, the safer you’ll be, and in the long term, I’d wager the happier you’ll be as well.

David Farquhar is a computer security professional, entrepreneur, and author. He has written professionally about computers since 1991, so he was writing about retro computers when they were still new. He has been working in IT professionally since 1994 and has specialized in vulnerability management since 2013. He holds Security+ and CISSP certifications. Today he blogs five times a week, mostly about retro computers and retro gaming covering the time period from 1975 to 2000.

The one extension that I load on the default browser of every computer I touch is Adblock plus (or an equivalent), with the privacy and web tracking blockers turned on as well as the default ad blocking list.
Not just because surfing with ads is simply a miserable experience, bur because of all the drive-by spyware and malware that turn out to be getting injected via poorly vetted ads.
Agreed, with Glaurung also. I’ve always figured learning to use the OS and keyboard is the best way to go, then you can always get around on another computer (usually to fix it) regardless of their preferred add-ons.