If this ever happens, your best bet is:
1) Try saving the file.
2) Then close Fireworks (wait until the Fireworks.exe disappears from Processes).
3) Start Fireworks again.
There's no other way of fixing this problem, I'm afraid...
Usually the error means there's a problem with Fireworks handling memory and it also means the app has been open for a long time and quite a lot of files have been edited (or maybe one complex file, with a lot of undo steps saved in temp Fw History).
Hope this helps a bit!
Btw, a general stability tip:
1) Set number of UNDO steps in Preferences to about 100-200 max.
2) Restart Fireworks CS4 or CS5 every few hours, especially after doing some complex work with one or more files simultaneously.
Once I adopted this workflow, I never saw this NOT ENOUGH MEMORY message again. Looks like there's a limitation in Fireworks core -- the app probably is not handling memory in an optimal way, especially after a lot of work with it..
I get the 'Not Enough Memory' warning all the time.
I've got number of undos set to 20 and I have to restart the programme every hour.
I've seen a number of complaints about this issue on different forums around the web: why Adobe ignore the problem is beyond me.
If I was involved in the development of this software, I'd be embarrassed to have it shipped with such a glaring fault.
Sort it out Adobe.