As soon as maximum contiguous virtual memory approaches zero, the browser will be automatically restarted to prevent "out of memory" (OOM) crashes (with either "mozalloc_abort(char const* const)" or empty crash signatures).
This is different approach comparing to measurement of occupied memory; this allows to make restarts only when they are truly necessary. This method is the only working one to avoid OOM crashes in 32-bit browser, because memory usage per se can be far from the limit of addressable maximum that 32-bit FireFox allows, but OOM crashes would still happen: after some point contiguous virtual memory deteriorates without dependence on whether you close tabs, clear closed tabs history and minimize memory usage through browser's procedure for that, or not.
This extension only works in versions of browsers that have "vsize-max-contiguous" measurement of memory state (check "about:memory" report; this measurement should appear near to "vsize" value): pre-release versions of FireFox 27 and above.
"Prevent Out Of Virtual Memory Crashes" is only needed for power users with heavy sessions in 32-bit build of FireFox for Windows. This extension will lose its usefulness as soon as Mozilla will resume support of 64-bit builds of FireFox for Windows. Of course, it is possible to have maximum contiguous virtual memory approaching zero even in 64-bit build, but it would be a rare case (for example, if you only have 8 GB of RAM and turn swap file off).
Based on http://addons.mozilla.org/firefox/addon/memory-restart/. Please follow the link and support original developer. This original extension tracks memory usage, which is, as explained above, not necessarily helpful if the goal is to prevent OOM crashes. Toolbar icon courtesy of Ken Saunders (http://mouserunner.net/).
All of functionality of the original extension is retained (as of version 1.15 of "Memory Restart"). However, memory usage threshold by default is set to 10000 MB, so it would not trigger restart before maximum contiguous virtual memory approaches low level.
By default auto-restart setting is on, because it is safe function even if you, for example, edit a form right at the time restart is happening: the latest version of the form is going to be restored after restart. However, if you have long time uploads in one of pages and/or do not want restart to be automatic otherwise, then in options you can switch auto-restart off.
Additional features: when approaching to the limit threshold, the extension checks memory state more often to more accurately present browser's current state. There is also intermediate state when extension's icon bubble is not already green and not yet red: it is yellow so you would know that the automatic restart (if the option is not turned off) will be executed soon.
Even though FireFox's method of determining memory usage should not impair operational fluidity in a significant way, it is not recommended to set refresh interval to low value.