Hi npub19rehaee8ww3gzrla523a2ajmgwdsx0arxrs3sutdryl9rlp9kxnq4kj2n6 (npub19re…j2n6)!
Maintaining extension support is complicated and unlikely to be added to Vivaldi.
What kind of extension(s) would you like to have? Perhaps there's a built-in feature your goal can be achieved with.