Fontviewer doesn't work here with the latest Java. The only difference is that I'm using a cmd file to add the java directory to the front of %PATH%.
As for SeaMonkey, in the old days, I'd say something in prefs.js but now a days it could be in one of the sqlite3 files, probably places.sqlite, which no longer seems to get finalized when shutting the browser down.
You could test. After backing up your profile, with SM closed, try replacing prefs.js with the new one and test. If it works, then you have the fun job of diffing the old and new prefs.js. You can do the same with places.sqlite (move places.sqlite-wal along with it) but it is hard to diff them and with the wal file, you can't even open the sqlite3 file in a sqlite editor. The wal file is sorta a journal, stands for write ahead log. Sometimes using an older SM (2.21 or older) will fix the sqlite file. Make sure you have your profile backed up. I back mine up daily due to problems like this, keep losing some history or not having it stick for some domains.