I use Firefox to edit Wikipedia pages, some form of Wiki markup, and I use syntax highlighting. Often the higlighting disappears and I get this message (below).
When I edit, I usually have only one other program open (Tex-edit Plus, not a memory drain) and one tab open. I have all but one Firefox extension disabled. 3 GBs of RAM in my iMac. Still I get a low-memory message.
Syntax highlighting on this page was disabled because it took too long. The maximum allowed highlighting time is 50ms, and your computer took 63ms. Try closing some tabs and programs and clicking "Show preview" or "Show changes". If that doesn't work, try a different web browser, and if that doesn't work, try a faster computer.
This is on Firefox 39, the most recent I can use.