We are currently shipping 5 lightweight themes with Firefox that take up 1.69MB on disk. LWT themes "3" and "5" are being shipped, but are no longer referenced by the lightweightThemes.recommendedThemes preference. In addition, theme "4" is using fairly large png images. 4.header.png alone is 769k. If were able to remove themes "3" and "5" from shipping, and were able to convert the png images to jpgs, we could reduce the total size of shipped lightweight themes to 615k. This would reduce the overall download size of Firefox by about a megabyte. Unfortunately, it is currently not possible to simply remove these themes or modify their resources. If a user is using one of those themes and updates to a new build with the theme resources deleted or moved, Firefox is unable to find them.

(In reply to Chris AtLee [:catlee] from comment #0) > Unfortunately, it is currently not possible to simply remove these themes or > modify their resources. If a user is using one of those themes and updates > to a new build with the theme resources deleted or moved, Firefox is unable > to find them. We could use a UI migration to specifically address this (by rewriting the user's prefs on upgrade) if we simply move/re-encode the images. To solve this bug as per summary, we'd need to update the dataformat we're using for lwthemes to separate what's stored in the prefs from where to find files for builtin system themes. I don't have a good idea for doing the latter. I also don't know if/how this will all be affected by the new webextension themes. Jared, can you comment on that?

The new webextension themes use the same backend implementation that the original lightweight themes used. Webextension themes are merely another API for lightweight themes that add more functionality. The UI migration step that Gijs mentioned would work if we wanted to move/re-encode the images.