i understand your worries
but the extension is full open source and you can review the code for easy on
https://codeberg.org/daskashk/lumo_ai_by_proton_chat_in_sidebar
whit a clean and easy to read basic code and MIT license
https://codeberg.org/daskashk/lumo_ai_by_proton_chat_in_sidebar/src/branch/main/LICENSE.md
i create a make file so you can build by yourself with easy and verify hashes with packages provided to verify is the same base code
the app is actually thinked to use as guess user, so is your responsibility to set your account, but there is no actually evidence of this extension being malicious, everyone can verify bc the4 code is full public and also i send the code to mozilla reviewers with instructions to make a reproducible build
i take security very serious, if you find any weird think please open en issue on codeberg to solve quickly

also the extensions say very clear in the 3rd line "This unofficial extension allows you to open the Lumo AI chat interface on the sidebar, similar to the Microsoft Edge Copilot sidebar. With Lumo AI's help, you can use AI without compromising your personal data or privacy." you dnt have to read much to notice is an unofficial extension
but if thats not enough it show this very clear:

⚠️ Lumo AI and all associated icons are the intellectual property of Proton AG . This extension is not officially endorsed by or affiliated with Proton. Icons are used solely to identify integration with Lumo, respecting their trademark.

⚠️ All my works are make with AI assistance as my description declare.

thanks for your reply, i hope this help you

EDIT: i add this to the short description to make is unofficial state more clear
"A Firefox Extension to run Lumo AI Chatbot by Proton web page on your sidebar. (Unofficial)"