Tree Style Tabs in Firefox are a killer feature to me. They allow to group and structure dependent tabs, and quickly collapse or close entire groups unrelated to the task at hand.

Also, typing % and then some text on the search bar allows to find a tab if you remember its name. But I use an extension that does a similar thing but shows multiple matching tabs and allows to close duplicates right from the list.

Many tabs mean much RAM use. Auto Tab Discard is essential to make a hundred or two tabs manageable.