Firefox 155 beta adds QUIC version 2 and two WebAssembly proposals, and moves the captive-portal check to a new domain

Sources: Firefox 155 release notes
Summary: The Firefox 155 beta release notes record 155.0beta as first offered on 2026-08-17, seven days before this digest, so this is not a same-day change. The notes add support for QUIC version 2 and for two WebAssembly proposals, which they name as Compact Import Section and Wide Arithmetic. The captive-portal detection check moves from detectportal.firefox.com to firefox-portal-detection.com. The notes also cap CSS nesting at 75 levels of nested blocks and functions, and narrow support for ::-webkit-scrollbar.
Why it matters: Network allowlists and captive-portal appliances that name detectportal.firefox.com need the new host added, and the 75-level nesting cap and the narrowed ::-webkit-scrollbar support are behaviour removals rather than additions.