I like the web UI, but if a desktop mail client crashed this often, I'd have deleted it long ago and switched to a different program. With webmail I don't have that choice.
Through a web browser is not the only way Fastmail offers to use its services. It also supports IMAP, so you could use any IMAP-supporting mail client (which is probably all of them).
Also, while you're waiting for them to fix this Safari bug you could try using a different browser.
I'm sure they'll fix the Safari bug eventually, I'm not worried about that. It's just an illuminating example of how difficult it is to create high-performance web apps that actually work in more than one browser.
Out of curiosity I once did a process sample with Activity Monitor from the crashed tab. Looked like WebKit was stuck in a text layout loop.
If I were their dev I'd suggest first, use another browser for your sanity and second, get in touch with the support team to see if you can help improve it!
> "This is the first I’m hearing about this — would it be worth it to respond and ask the user to reach out to support?"
So yeah - please do contact our support team (support@fastmail.com or you can use the web form at https://www.fastmail.com/support/) and give as many details as you can about what you're doing when it happens!