I went back to Firefox 17 ESR (Extended Support Release) and suddenly the tests came back to life (using Selenium server 2.33.0).
This problem seems to be an indicator for version incompatibilites between Selenium server and Firefox. Unfortunately, there's not much, or anything, to be found in the documentation on this matter - but then again, as @Pavel Janicek mentioned, Selenium RC is deprecated. What I've been able to grasp from the CHANGELOG is that Selenium server 2.32.0 explicitly supports Firefox 10 ESR, 17 ESR, 19 and 20. For 2.33.0, no additional browser versions are mentioned, leaving Firefox 21+ officially unsupported, although not everyone agrees.
To wrap it up: If you see this error, the only solution may be to step down the list of supported browsers in the current CHANGELOG until your tests run again.