The biggest problem with gauging how Firefly stacks up next to Buffy or Angel is that Firefly never got the chance to develop. It'd be like judging Buffy by the first half of season 1.
Firefly, by its nature, was a lot more open ended and geared towards anthology stories than the earlier Whedon shows. But... it had plot threads, and it had the beginning of a central theme. It just never had a chance to go anywhere, and the movie largely focused on trying to create a sense of resolution and "sailing into the sunset" to cap the aborted first season off.
For example, one potential theme in Firefly that was hinted at, might've been a long an in-depth examination of the nature and purpose of belief. The character of Mal was set up to be a former man of religious faith, who had been turned entirely against not just religion but the very notion of belief itself. The movie presented a hyper rushed wrap up with Shepherd Book telling Mal "faith" had nothing to do with God and was about what believing in something did to a person, with the enemy in the film a counter example of belief being used for evil.
I always suspected that the show was going to have some very interesting themes, especially coming from Joss Whedon's own position as an atheist who would likely be inclined to agree with Mal in his fallen-out-of-faith state.
Then there was the complex, morally shades of grey scenario that was being built with the overt struggle between the Alliance and the people they oppressed. Because the Alliance wasn't simply evil, and the people inside the Alliance generally were not Nazis and even had legitimate reasons for believing what they were doing was right. This would have been a big change for a Whedon show where, while the cast usually deals with grey morality, they have a pretty simple, cartoonish Big Bad to fight, and the forces of evil are literally supernatural eternal evil.
In Firefly, the "big bad" was just life and its messy complexity.