If you start out with one or two and don't add any more for a while, rather than loading a bunch at once, you can more easily isolate a problem. It would likely be the most recent extension you installed.
Pipelining is tweaking the browser so it's faster. There are a couple of extensions that do this, but it can be done to extremes, swamping sites' servers. This one,
Tweak Network, is simple, effective and not rude to servers. You'll notice a speed increase.
SmoothWheel was the first extension I discovered, way back with OS 9, and it put me off all other browsers but Mozilla's from that moment on. Scrolling is as smooth as screen credits sliding past in a movie. This link is to the author's home page. The version listed at Mozilla's extensions site might be too old to install in the most recent Firefox.
You could try SmoothWheel the way it arrives. If scrolling it too slow, go into its preferences, click on advanced and fiddle with its controls. At the moment in Advanced, mine's set to High; High; 5 steps; 1/3 step. (The faster I use the wheel, the bigger the page-scroll jump.) The top of the download page has a link to the SmoothWheel Home page that explains it.
Right now, I'm running Adblock (
not Adblock Plus that can litter pages with little Adblock labels
and not Adblock Filterset.G Updater (which, itself, should not be used with Adblock Plus), AniDisable, BlockSite, Clear Private Data, CustomizeGoogle, DownThemAll, FastDic, Firebug, Full Screen, Menu Editor, MIME Edit, Mouse Gestures, MR Tech Local Install, NoScript, OpenBook, PDF Download, PrefBar and User Agent Switcher.
Some come and go. In the process of writing this, I deleted a few I never bother with, such as FoxyTunes. I always forget it's there.
This link is to the Mozilla site extensions. Another, less well-known, is
here. There are extensions at this site not listed on Mozilla's. Contrary to appearance, you don't have to log in. At the top left there's a link to Firefox extensions, and there's a search box farther down.