As someone firmly on the Left...

Dangling modifiers are one of those 'grammar' things that children learn about, writers are warned about, and peevers peeve about. Strunk and White give this one:

Being in a dilapidated condition, I was able to buy the house very cheap.

They're also one of those grammar things, like so many of these peeves, that actually don't usually matter much. It's nearly always clear from the context what is meant, and maybe the sentence would be neater if it was changed, but not always. The one above is corrected to 'Being in a dilapidated condition, the house was for sale very cheap' which is fine too, but I don't think there's much in it, readability-wise.

So it was quite exciting to find one that actually does matter:

As someone firmly on the Left, Jeremy Corbyn would be a truly terrible Prime Minister, even in comparison to the appalling one currently at No. 10.

— Mary • Pronouns: Hufflepuff/Esquire ❤️🐱🏁🕷 (@MaryMinifieEsq) October 26, 2019

The dangling modifier here is 'as someone firmly on the left'. It can refer to the speaker, as these modifiers often do, or it could refer to Jeremy Corbyn, the closest referent. This is the interpretation that style guides would say is 'correct'. But both are plausible, which is why we can't disambiguate with context here. The two different meanings are as follows:

I am someone firmly on the Left, but I still think that Jeremy Corbyn would be a terrible Prime Minister. 

Because Jeremy Corbyn is firmly on the Left, I think he would be a terrible Prime Minister.  

I actually don't know which is the intended meaning, as I don't know the person who tweeted it. Probably the first is more likely given the audience and tweet it responds to, but who knows. A rare genuine case for writing out the dangling modifier.