Firefox still gets slow over time, and has done so in every version I've ever run going back years. I'm currently running Quantum 67.0.4 64 bit on a Windows 10 machine that is completely updated and has plenty of RAM. I have three extensions installed -- UBlock Origin, Clip to Evernote, and Bookmark Dupes. That's it. I have downloaded themes, but am not currently using one unless the "light" theme (that used to be the default and still should be by the way) counts. I've decided that I am not doing the re-set/uninstall everything cha-cha-cha. Firefox touts its extensibility but at the first whiff of something wrong the advice is uninstall every extension you love using. If I do that, I might as well use Edge, which at the moment appears to be twice as fast in accessing pages, as is Chrome. Firefox is my browser of choice, and has been for some time, but I'm afraid for not much longer. Mozilla needs to fix this once and for all.