It seems that you might be able to edit the dictionary file in a text editor, but I haven't tested the theory:

C:\Program Files (x86)\Mozilla Firefox\dictionaries\en-US.dic

Changes here probably won't survive the next update, but if you only need to make a couple edits, it might not be so bad re-doing them every month...

(I'll try a test edit and see what happens.)

Yes, editing that file works. Because the line breaks are not traditional DOS line breaks, you should not use Notepad as your editor. Instead, try this:

(1) With Firefox closed, open the Start menu to All Programs > Accessories

(2) Right-click Wordpad and choose Run as Administrator, give yourself permission when prompted by Windows

(3) Navigate Wordpad's Open dialog to C:\Program Files (x86)\Mozilla Firefox\dictionaries and change the file type to all files (or type *.* in the box and press Enter) so you can open en-US.dict

(4) Search in the page (Ctrl+f) for dong/ (it's not the first one) then add XYZ between dong and /

The reason for this is so you can easily find and undo your changes by searching for XYZ/ or globally replacing that back to / if needed.

(5) Save and then restart Firefox. If spellcheck seems to work correctly, you can close Wordpad. If not, you might want to undo your changes in Wordpad (you might or might not be able to save that with Firefox open).