I’ll copy-paste this stackoverflow answer, because it sounds like its written by someone more knowledgeable than me.

blobs are binary firmware, not distributed as source. they are necessary for certain hardware drivers to function, in whole or in part. As the source is not readily available, there are deblobbed distributions for opensource “purists”.

In the firefox context, there are certain non-free portions of firefox code base, namely the branding and some mozilla services integrations. Those are removed. This is more about licensing then availability of the code.