fix(theme-classic): restore right padding on sidebar (DocSidebar) in Firefox

[epifaniofrancisco]

Pre-flight checklist

• [x] I have read the Contributing Guidelines on pull requests.
• [ ] If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• [x] If this is a new API or substantial change: the PR has an accompanying issue (closes #11130) and the maintainers have approved on my working plan.

Motivation

Added @supports (-moz-appearance: none) and (scrollbar-gutter: stable) to set padding-right: 0.5rem for Firefox-based browsers, fixing missing right padding on sidebar while preserving behavior in other browsers. The issue occurs because the existing @supports (scrollbar-gutter: stable) block sets padding: 0.5rem 0 0.5rem 0.5rem, which removes right padding in Firefox-based browsers (e.g., Firefox 137, Librewolf 137, Zen). This fix uses -moz-appearance: none to target Firefox specifically.

Test Plan

1. Run locally:
   • Clone the repository and run yarn install.
   • Start the development server: yarn workspace website start.
   • Open http://localhost:3000 in Firefox 137, Librewolf 137, Zen, Chromium, and Epiphany.
   • Inspect the sidebar buttons (.menu) using Developer Tools to confirm padding-right: 0.5rem in Firefox-based browsers.
   • Verify that Chromium, Edge, and Epiphany retain correct padding (no regressions).
2. Run tests:
   • Execute yarn test to run unit tests and confirm no failures.
3. Screenshots:
   • Before: Sidebar buttons in Firefox 137 lack right padding.

• After: Sidebar buttons in Firefox 137 have padding-right: 0.5rem.

4. Browsers Tested:
   • Firefox 137, Librewolf 137, Zen (fixed).
   • Chromium, Edge.

Test links

Deploy preview: https://deploy-preview-_____--docusaurus-2.netlify.app/

Related issues/PRs

• Issue: #11130

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/	🔴 46	🟢 98	🟢 100	🟢 100	Report
/docs/installation	🔴 48	🟢 97	🟢 100	🟢 100	Report
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/blog	🟠 57	🟢 96	🟢 100	🟠 86	Report
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[slorber]

See https://github.com/facebook/docusaurus/issues/11130#issuecomment-3139676727