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.