Using XMLHttpRequest POST to a mod_rexx script on the server works for all browsers except Firefox. Firefox will always append "charset=UTF-8" after the Content-Type request header as seems appropiate as per spec of the W3C. Other browsers don't seem to do this yet.
Mod_rexx will then fail to parse the request string into the WWWARGS. stem due to a restrictive strcompare in rxfuncs.c in the WWWGetArgs function at line 513.
Changing the compare to only compare for the length of "Content-Type" disragarding the appended character encoding will fix this for Firefox.
Same thing is true for mod_oorexx, but since I'm unable to compile mod_oorexx cleanly for my Centos system i'm still using mod_rexx.