If you are in a legal/political environment that allows active attacks by an ISP on their customers then you have a much bigger problem than just this sort of thing.
What can I, an individual with nearly zero political clout (I can vote... but I live in a gerrymandered district... another layer to the problem!) do to get this problem fixed? Best case scenario, I say something, it goes viral, people march in protest against bad ISPs, the monopolies get broken up, etc. But how long does that all take?
Meanwhile, I'll only use HTTPS websites as a first-line of defense. The "much bigger problem" can be solved in parallel.
Isn't it then possible, albeit harder to do a MITM attack at this point with HTTPS?
If the "firewall or other device" tries to interpose when you don't trust it, that won't work, for the same reason it doesn't work for bad guys. We do not distinguish the apparent reason for attempts to destroy the confidentiality, integrity or authenticity promises in TLS, whatever the apparent motive your HTTP transaction will either work, delivering those promises or you get an error and no progress is made.