Samphan Raruenrom has produced a Firefox extension based on ICU to handle Thai line breaking.
Thai line breaks respect word boundaries, but there are no spaces between words in written Thai. Spaces are used instead as phrase separators (like English comma and full stop). This means that dictionary-based lookup is needed to properly wrap Thai text.
The current release works on Windows and the current Firefox release, 220.127.116.11. The next release will also support Linux and will support future Mozilla Firefox/Thunderbird releases.
You can test this on our i18n articles translated into Thai.
This replaces work on a separate Thai version of Firefox.