Full-width spaces in Subject are replaced.

[yoshiaki_at_arc]

When a full-width space (U+3000) is contained in a Subject, it is converted to a normal space (U+0020).
For example, “あ　い” becomes “あ い”.
For Japanese, U+3000 and U+0020 are different characters.
The same thing happens when saving to draft.

[siffemcon]

You could report it as a possible bug (https://bugzilla.mozilla.org/enter_bug.cgi/).