docs: document Firefox base-select support

[KirtiRamchandani]

Problem

#44299 asks for Firefox documentation for appearance: base-select, which shipped in Firefox 149 behind layout.css.appearance-base-select.enabled.

Root cause

The appearance reference already documents base-select, but the Firefox 149 release notes and the persistent Experimental features page did not mention the Firefox preference.

Solution

• Add a Firefox 149 experimental-web-features entry for customizable select elements.
• Add a matching Experimental features table entry with the preference and channel availability.

Tests run

• npx --yes prettier@3.8.3 --config .prettierrc.json --check files/en-us/mozilla/firefox/releases/149/index.md files/en-us/mozilla/firefox/experimental_features/index.md
• npx --yes --package markdownlint-cli2@0.22.1 --package markdownlint-rule-search-replace markdownlint-cli2 --config .markdownlint-cli2.jsonc files/en-us/mozilla/firefox/releases/149/index.md files/en-us/mozilla/firefox/experimental_features/index.md
• git diff --check

Notes

A companion BCD update will be opened separately.

[pepelsbey]

Hey! Thank you for trying to help. These issues related to Firefox release notes are covered by MDN's content team and have a dedicated workflow, wider scope, etc. Feel free to pick up issues with these labels: good first issue, help wanted.