If you have too many open tabs - that means you hardly use bookmarks and don't use the history.

There are some extensions which would let you prevent garbage entries (like countless GMail, HN main page etc duplicates) from appearing in your history (by means of auto-deleting them as soon as they get there or just by showing you a filtered version of the history). Using such practically turns yout history into an endless resource-safe tabs list.

There also is TidyTab Chrome extension (what's the Firefox alternative, by the way?) which would let you bulk-bookmark and close the open tabs.