On macOS with Apple M1 the Save as PDF creates huge files. As alluded to in a now closed question/issue with same title, Firefox apparently renders the file as an image. Using system dialogue to print results in the same large file size. An example .htm file with 2 embedded images saved as PDF from Firefox is 26MB (typically too large to email), while saving same file in Safari results in a 2.4MB file. My recollection is that the behavior is similar on Apple X86. This feels like a bug.