[Firefox] Text selection does not work with mentions.

[shved270189]

Steps to reproduce:

1. Open a page with the Lexxy editor in Firefox.
2. Add some text and a mention.
3. Select the content.

Actual behavior:

Content selection does not work.

Expected behavior:

The content should be selected.

Screen.Recording.2026-06-22.at.11.53.40.mov

[lyubomir-bozhinov]

I'd like to pick this one up alongside #1149 — I suspect they share an underlying cause.

Mentions render as DecoratorNodes (CustomActionTextAttachmentNode), i.e. contenteditable="false" elements inside the editable root, and extending a selection across such nodes is a long-standing rough edge in Gecko. src/editor/selection.js already carries both Chrome-specific and Playwright/Firefox-specific workarounds around [data-lexical-cursor], so the decorator selection path looks like the place to dig.

Will verify in Firefox and report back.