Getting fired is up to the employer, NOT up to you or me.  Their employers don't have a problem with it, so it isn't an employment issue.  If you choose to boycott the NFL, that is up to you, ultimately, if enough people boycott, the league and/or owners will absolutely put a stop to the protests.

As for arresting BLM protesters, how about arresting the bad actors, instead of trying to lump the entire group together with the few bad actors.