"With Send you can share file sizes up to 1GB quickly. To send files up to 2.5GB, sign up for a free Firefox account."
this is an interesting distinction; i'm not sure why there is a change for files over 1Gbyte. (perhaps the server mediates chunking up >1G files?) regarding the end to end encryption, as long as you trust third party Mozilla, and protect your well generated secrets, it is robust: https://hacks.mozilla.org/2018/11/firefox-sync-privacy/ (the same mechanism is used for Send as for Sync) keep an eye on https://www.mozilla.org/en-US/about/policy/transparency/
