Hi,
It seems that webmasters still care about files size for their web
pages, because they still minify their javascript and other things to
reduce band width usage.
Today there is a better alternative than PNG for lossless images in
terms of file size.
A .bmp image compressed with lzma usually gives a smaller file size,
as you can see with this example:
9,5K — logo.png
7,2K — logo.bmp.lzma
The lzma lib is a quite small lib, and reading a .bmp images is trivial.
Making Firefox able to read this compressed format would maybe not
require too much work.
The file extension could be .bmpz or .bmz (following the .svgz).
Best Regards
Florent Monnier