5.01.8 crashes with Microsoft IME in Japanese mode

[japanat]

I'm running the new Win10 beta 5.01.8 64bit. It works fine with Firefox when using US keyboard or when using MS IME in English character mode. When I try to switch to Japanese characters, it freezes, then crashes about 1 minute later.　No error messages, just freeze and crash.　MS IME works fine in unsandboxed Firefox.

Any ideas? I don't want to install other IMEs if they aren't necessary, but will if I must. I just feel naked w/o the sandbox, and positively dread my son getting on my PC w/o it...

Win10 64bit
Firefox 40.0.2
Sandboxie 5.01.8
Microsoft IME 日本語