Fingerprints have a different and weaker legal standard than passwords to protect them
> Use an Android phone.
It may be possible to get a secure Android phone, however, it is unlikely that the one you have is. Varying levels of quality for disk crypto and TPM key storage will do you in.
> Take the devices you work on across the US border
Any data or passwords you have on you is data you could lose, get forced to cough up, etc.
> Assuming I have to carry my laptop and phone across the border, what precautions can I take to minimize the potential privacy violations?
Put an encrypted blob on [name a cloud provider]. Download it once you cross through customs.
> why is Firefox not recommended?
Because Firefox has no sandbox and gets routinely exploited by Law Enforcement
> It's used in the Tor browser
The Tor Browser is an abomination.
> I have not heard of any major security incident recently with Firefox.
You have not been paying attention. Maybe consider accepting the advice of experts?