test: the Firefox suite tolerates no extension error (closes #487)

The one entry in ALLOWED_ERRORS in tests/e2e/firefox/run.js, Firefox
reporting a popup promise that settled after the page unloaded, lost its
cause when the site-connection buttons stopped sending with an unawaited
sendMessage before closing (#275). Left in place it would also hide the
same error from any other popup code that sends and then closes. The
entry goes, with the code that only printed and set aside tolerated
errors, and the README paragraph that described it.

Model: opus-5-5

TODO.md

@@ -45,6 +45,14 @@ but the review is broader than any of them.
 
 # Completed Steps
 
+- 2026-10-07: The Firefox end-to-end suite no longer tolerates any uncaught
+  extension error ([#487](https://git.eeqj.de/sneak/AutistMask/issues/487)). Its
+  one entry, Firefox reporting a popup promise that settled after the page
+  unloaded, had lost its cause with
+  [#275](https://git.eeqj.de/sneak/AutistMask/issues/275); the entry and the
+  code that printed tolerated errors are gone from `tests/e2e/firefox/run.js`,
+  and so is the README paragraph that described it.
+
 - 2026-10-07: The toolbar icons in `icons/` can be redrawn in the tree
   ([#378](https://git.eeqj.de/sneak/AutistMask/issues/378)): `make icons` runs
   `script/lib/icons.js`, which draws the mark and writes each PNG with node's