So, I have all my tags in foobar2000 as id3v2.4, and it all works great. Until other programs like to spit it out (not showing Genre correctly, etc.) I also have several Custom fields that I use to organize my MP3s. I'm pretty sure I lose all these fields if I use Compatibility Mode (2.3), can someone confirm this, and is there any workaround. (Also, why don't people make their programs compatible with a technology that is YEARS OLD?)

No, you won't lose your TXXX frames with the most recent foobar2000 version.