Edit: OP, if you read what I wrote, you will see why you shouldn't use the number of upvotes to make your decision: the point is precisely that a 'popular' choice can quickly turn south. At this point, all these upvotes will have disappeared and you will be alone facing the consequences.
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If they did something so awful that it's keeping you awake at night, do it.
Otherwise, don't.
...
Wait... are you still reading?
Don't!
....
Okay, darn it...
If you STILL want to do it:
Career-wise, make sure it's something which is universally seen as awful AND will probably continue to be seen as such. If the wind changes direction, there's still going to be a record about what you've done.
Things seen as obviously right can be seen as obviously wrong in the time of a single decade.
When this happens, the 'major law firm' and all those who did encourage you to do it will magically vanish.
> Would it be a deal killer if I applied at your company?"
Depends on what and why you did it.
If you testify only because someone broke the law, from my point of view as a hypothetical employer, you are a huge liability. Most people break the law multiple times a day and don't even know it. I honestly don't want people around me who are ready to report me just 'because I broke the law'.
If you've done it for moral reasons, even if I agree with you, I might have to refuse to hire you because it might gives a bad image to my business.
Now, what if I agree with you and you did something which is currently politically correct. Is there any advantages? Not really... I still don't know if you are good at your job; I'll have to evaluate you the same as everybody else.
In short, there's no advantages (employer/employee-wise) to doing it. Only potential downsides.
It's like behind an asshole on a social network. It might gives your friend a few giggles, but it can hurt you years down the line.