Not going to lie, the hours can be crazy in tax, but they can be more manageable too.
With audit, there's an expectation of travel, and early starts no matter where you are, i.e. if you have to travel to a client site and it's much further away than your office, you have to suck up the longer commute. There's much less of an expectation to travel in tax, which immediately cuts down on the long hours point.
Like audit, tax has certain sectors that are more demanding. FS, as mentioned, can be tough, especially in a larger firm. It's all to do with the sort of client and their expectations, so it doesn't matter whether you're the auditor or the tax person, the same sort of people will be the same sort of (un)reasonable to the professional help.
I am indeed a tax person, although I have been trying my best not to be biased!
I think the key thing to remember is that you do have options. Being fired can send people into a right tailspin - they panic, they feel miserable, and they get too caught up in those negative emotions to notice that it happens to other trainees in accountancy too, and it really won't be the end of the world. You are so early on in your career, and there is so much more to come - you have to remember there is a long game here. You are still in the game!
I've seen people decide accountancy isn't for them, and start a new path. I've seen plenty more decide that it is what they want to do, and they've gone onto finish their qualifications elsewhere and do amazing things. Some have even levelled up, and gone from a small firm fail to finish off at a larger firm. (Not that applicable to you, seeing as you can only go sideways to another Big 4 firm, but you get the point.) If you do want to stay in accountancy, getting fired in your training contract doesn't have to dictate the rest of your career.
As for ajj2000's advice - ICAS are currently on a drive to persuade people to train to be CAs, given they're vastly outnumbered by ACAs in the UK. You might be able to tap into that sentiment by asking for help in staying with ICAS, and not having to transfer to another qualification. There is an email address for students.
I wouldn't suggest spamming LinkedIn, but some people do have things on their profiles saying that you can contact them for coffee and mentoring. If you spot anyone potentially useful in your search who is open to that sort of thing, you could send a short, polite message explaining you are a current CA student at a crossroads and you'd really appreciate it if they could spare 15 minutes for a virtual coffee.
This may have passed you by, but ICAS have recently launched a student assistance plan, which I gather is like the employee assistance programme most firms offer, but provided by ICAS to its students, i.e. you can continue to use it even after you get fired. They can help with all sorts of things - legal queries etc - but also counselling if you end up feeling in a bad place because of what's going on. It may not come in useful, but I thought I'd draw your attention to the service just in case!
I hope all this does help you refocus your efforts and figure out where you want to go with your path. It's been a long time since I was on a training contract, but I can remember how overwhelming it felt at times, and if you need a sounding board once you put together a couple of options, you're welcome to send me a PM.