And Firefox Send has much higher limits too, for free. Without a Firefox Account, the user can upload up to 1GB. And with a Firefox Account, the limit is 2.5GB. Dropbox Transfer, in comparison, provides (just) 100MB in the free tier.

Yea but it doesn't seem like Firefox Send lets you right click on any file anywhere on my computer and just right click to Transfer. The uploading with Dropbox Transfer (through the desktop client) is super reliable too -- I have Time Warner internet and when it invariably craps out, my upload auto-resumes. Looks like this here: https://www.dropbox.com/t/jJNe5IBPrezaw7aE

As an existing Dropbox user, Firefox Send or WeTransfer is not as good a product for me. Because I already have the Dropbox client installed for file storage, I don't need to visit any websites to upload to Transfer. I can literally just right click on any file anywhere on my laptop and click "Transfer" -- then it automatically pops up, I can add more files into it, rename it, etc. and then just send an email or copy link. I screenshot how it looks here https://www.dropbox.com/t/jJNe5IBPrezaw7aE (and it's so easy I find myself just automatically use it). What I really like about it is that it sends a copy of the file and my grandma won't just delete my photos like the good ol' Dropbox links.

Is visiting send.firefox.com that much of a friction?

Compared to the dropbox option, yes.