I released a new Skippy today, version 1.3.3. Ingvar exposed a bug by using the skippy API to write out GIFs with zero-length comments. These are prohibited by the spec:
iii) Comment Data - Sequence of sub-blocks, each of size at most 255 bytes and at least 1 byte, with the size in a byte preceding the data. The end of the sequence is marked by the Block Terminator.
Skippy's old behavior introduced a spurious terminating block into the stream, and Firefox rightly refused to display the resulting images. Skippy 1.3.3 will not produce these bogus GIFs.