We deliberately did not develop a browser plugin:
- If we develop one like MetaMask, that would involve key management in a browser / browser plugin environment, which isn't as secure and isn't as private as a smartphone.
- Smartphones come (these days) with features browsers & desktops don't ship with: biometric security (Face ID / Fingerprint scanner), NFC reader (e.g. for hardware wallet, like Tangem cards)
- Browser plugins usually don't work on smartphones, and huge part of the general crypto and web use today is mobile based, especially in Asia / South America
- Browser plugins are very susceptible to scam/phishing/... attacks. They can be faked (rebuild, mocked) in a webpage
- And most importantly: the interface with XUMM (mobile) is as easy as scanning a QR code to be displayed in any web environment: that's as easy as, if not easier than opening and unlocking a browser plugin.
One can still use all XRP Ledger features in a user friendly way, signing with XUMM, without needing a browser plugin. For more info (tools) also read: Is there a desktop version of XUMM / will there be a desktop version?