I cannot be 100% certain but I'm confident I was using it not long after the 1.0 release. That'd put me at 2004. 19 years!
Although I did briefly switch over to Chrome when it was new and fast. Then switched back when Firefox had a major optimization pass.
There was one extension I used in Chrome that I haven't found a Firefox replacement for, but I stopped trying to look a while ago and just live without it.
Was a specific kind of cookie manager: you could whitelist a set of websites to keep their cookies. Everything else would be deleted when you told the extension to do so.
Too many websites need cookies that stick around indefinitely. But I also don't want to delete everything everytime I close Firefox, because I may want to keep a website around for a few days without wanting to bother adding it to a whitelist.