This error is followed by a number, which indicates how many textures have been discarded. Once it appears, you'll find that other textures will fail to rez in, and in many cases, shortly after the error message appears, you will crash out.
The message is fairly self explanatory; it means you have run out of available memory. Now some say, “But I have 16GB of RAM, isn't that enough?” Well, especially with 32 bit viewers, you won't be able to use all of that memory. Depending on your operating system, only 2 GB or 3 GB are available to the viewer.
Some also note that older versions of Firestorm never had this problem, that they never saw this message so it must be a new bug. It is correct that older versions didn't show this, but that doesn't mean that the problem didn't exist. Rather, no message was shown and you would simply crash. The addition of the message at least indicates what the problem is, and thus makes troubleshooting it easier: rather than trying a whole host of fixes which might be irrelevant, it is narrowed down to just a few.
If you have been using Firestorm for some time, and only got this once, or rarely, then it probably happened because you were in a very graphics intensive location. The “fix” therefore is to lower draw distance before going to such places, and keep it low till you leave.
Be aware that the message, once it appears, will not simply go away; you will need to relog. This is advisable anyway as new textures will most probably fail to rez.
In that case, Firestorm is trying to use more memory than you have available (remember the 32 bit limitation mentioned above). You need to reduce memory usage. For example: