I first became aware of and started using add-ins due to the Masterworks board. Someone there suggested "Ad Blocker", as it was the ONLY way to stop repreated, incessant VIDEO ADS WITH SOUND playing every G**D*** time I'd try to go to their message board.
I also found "NoSquint", which allows me to adjust the width of each site I go to separately. With the advent of "wisdescreen" monitors (which I DON'T have), every idiot on the planet is designing websites to look like CINEMASCOPE.
And in line with that (sort of), there's also "Image And Flash Blocker", which allows me to shut off all images (and turn them back on easily) which comes in handy at places like Masterworks, when people post images that are so big they make it impossible to read the messages. (And someone at one of the boards asked, "Why are you posting links? Where are the images?" Some people are real idiots...)
Nick Simon (who had the SILVER AGE MARVEL site... until it went down) also told me he always checked his websites on multiple browsers. Then again, he also designed everything with dial-up in mind, since not everyone has hi-speed (YES, EVEN NOW, I know some people who don't!!!). This is one reason I try to limit the big images on my blog pages to 10, and have the default for how many pages come up when you click "Home" on the blog to 1. So many people wil have "5" or "10", and sometimes, EVEN WITH hi-speed, it can take several minutes for some blogs to come up that way. It just overloads your temporary memory, among other things.