When using Firefox on her computer, my wife opens a new tab when starting a new search, and opens links in a new tab when she may want to use the linking page later.
However she never closes tabs, effectively using them instead of bookmarks, which causes memory usage to keep growing, then swap use to keep increasing as tab count goes into the hundreds. Later as memory usage becomes too high it is no longer possible to hibernate, and we eventually reach a state where the whole computer becomes unusable (for instance bringing Thunderbird to front and clicking on a mail will take several minutes during which the whole computer is unresponsive). At that point we need to either close firefox entirely (which may take up to ten minutes and may lose unsaved data) or reboot (same).
I added memory to her laptop (now 2G in total - it's an old model, that's the highest capacity I could find that was compatible) but it just means she is now able to open more tabs before things start breaking.
My question: is there a way to extend or modify Firefox in a way to let her follow this usage pattern without memory usage exploding? I imagine merging the concepts of tabs and bookmarks, e.g. so that tabs that have not been opened for several days be automatically bookmarked and closed (going into the filesystem where it won't take precious resources).
Ideally, some pages such as the GMail web interface and currently playing YouTube videos (they can be recognised by the initial "▶") should never be auto-closed.