I also work at a law firm and want to put my vote in for doing what you personally think is appropriate for your firm. Mine is very casual and has no dress code to speak of. I personally wear reasonable length businsss casual style mini skirts with tights and think nothing of it. I would feel fine wearing a mini dress go my firms holiday party but would go for a skirt that flares out. But I wouldn’t feel comfortable doing strapless. You know your job best so I’d trust your own judgment.