I use Tab Center (now Tab Center Redux), which shows you the tabs on the sidebar (great for >20 tabs). The sidebar also has a search bar to search open tabs!
Besides that, by default (no addons or settings necessary) Firefox's awesomebar (the URL bar) will search open tabs if you type stuff. This is great because often I'll want to open a site and it will show me that I already have a tab open for that and let me choose to just switch there anyway. I am surprised Chrome doesn't have this yet, really.
https://addons.mozilla.org/en-US/firefox/addon/tab-center-re...
Besides that, by default (no addons or settings necessary) Firefox's awesomebar (the URL bar) will search open tabs if you type stuff. This is great because often I'll want to open a site and it will show me that I already have a tab open for that and let me choose to just switch there anyway. I am surprised Chrome doesn't have this yet, really.