I was doing a bit of experimenting just now with JWPce, still my go-to app for my Japanese text-entry needs.
Curiously, I can copy text from there and paste it in anywhere in Firefox (even the Topic subject line, as shown) and it will appear normally. Attempting to paste the same text into the message body here results in an auto-created image link, like , which doesn’t even seem to work properly.
Re-copying the text after pasting it somewhere in Firefox results in text that can be pasted into the message body normally, as in （１２３４）, so it seems that copy-pasting from JWPce directly is the only problematic operation.
Is this supposed to happen? Granted, copy-pasting from JWPce is hardly something people would want to do very often, but perhaps it is indicative of a potentially greater problem?