Some combination of Firefox updates in the 1.5.x range or perhaps Java updates had left Regex Powertoy in a state where it usually wouldn't initialize properly, leaving it non-functional.
It's been updated with a new way to delay initialization until the necessary background applet is surely available. Also, a couple bugs with replace backreferences and replace matchmarks have been fixed. If it's seemed flaky the last time you tried it, give it another whirl.
(For more background on Regex Powertoy, see this post.)