Physical memory is the memory that is installed on your computer as real memory.
To be able to run programs that require more memory and more programs at the same time (via time sharing and task switching) it is also possible to move (swap) currently not needed code and data to disk to make room for other programs and data.
The total memory available for programs is the Virtual Memory.

You can check the Virtual Memory settings here: Control Panel > System > Advanced > Performance > Settings (Advanced tab)
Windows can manage more memory then is physical available and that memory is stored in a page file on your hard drive.
If the size of that file is not large enough and all available memory is in use then you get the Virtual Memory is low warning.
Make sure that you have at least 1 GB or the recommended setting for the Virtual Memory.
If you have less than 1 GB physical memory installed on your computer then if possible add more memory.

• http://support.microsoft.com/kb/2160852 RAM, virtual memory, pagefile, and memory management in Windows
• http://en.wikipedia.org/wiki/Virtual_memory

I had the same problem. It has to do with memory allocation on your computer. Firstly, make sure you have enough RAM. 1GB should be good enough. You need to allocate more memory from the Advanced System Settings screen.

There is a very helpful Step-by-Step guide here How to fix Mozilla Firefox error - Virtual Memory is too low

It fixed my problem. I use Windows 7. If you're still getting the error, update to the latest version of Firefox or uninstall and reinstall Firefox.