So I finally got around to watching Igor's great video on how to try to reproduce Emacs bugs using a minimal configuration, etc:

With this in mind, I took a look at the related list:
https://list.orgmode.org/orgmode

I then found this particular bug:
https://list.orgmode.org/orgmode/m2ilowp4br.fsf@ntnu.no/

I then went through the motions as dictated by Igor's video. Ultimately, I was unable to reproduce the bug. I noted the instructions with the 'Reply Instructions' section.

I clicked the link which opened up a related Emacs buffer to which I pretty much replied that I couldn't reproduce the issue.

I then hit C-c C-c which apparently sent the email. However, I checked the bug itself, but cannot find my reply message present, and am wondering if it actually was sent at all. I personally had 'gnus' configured at one point but barely use it because I found it to be super awkward and clunky, and just not friendly to use in comparison so I just use the GUI client for Fastmail these days.

Here is my output of my '*Messages*' buffer in relation to this issue:
Sending...
Sending via mail...
Sending email  
Sending email done
Sending...done

Questions:
Q1. Is it possible to reply to bug threads like this within a GUI email client like Fastmail? 

Q2. Or is this only possible to directly reply to email threads if you're using mail clients like 'mu-4e', 'notmuch' etc?

I ask because I'll work on my email setup accordingly if that is the case where just a GUI client isn't sufficient as I was planning to use 'notmuch' eventually anyway, but wanted to confirm this before putting in the effort to do so.

The only thing I found was this article by Fastmail, but don't know if this would actually work with just the web browser client of Fastmail I use in Firefox:
https://www.fastmail.help/hc/en-us/articles/360058753594-Import-your-mail

Thanks,

Sam