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

README.md

@@ -603,14 +603,6 @@ without an `await`. Demonstrated, not assumed: a `throw` placed past the first
 and on Chrome (`pageerror`), with the rest of the run unaffected because the
 approval view had already rendered.
 
-One error is still tolerated rather than fatal, listed in `ALLOWED_ERRORS` in
-`tests/e2e/firefox/run.js` and printed on every occurrence so the concession
-stays visible in the run output: Firefox reporting an extension promise that
-settled after its page unloaded, from anywhere in the popup. Its cause was the
-site-approval popup's unawaited `sendMessage` before `window.close()`, which
-[#275](https://git.eeqj.de/sneak/AutistMask/issues/275) removed; Firefox runs
-since have not printed it, and
-[#487](https://git.eeqj.de/sneak/AutistMask/issues/487) removes the entry.
 Errors are read from the privileged `nsIConsoleService` in Marionette's chrome
 context and filtered to non-warning entries whose `sourceName` is the extension
 origin. That mechanism is not a stylistic choice. WebDriver BiDi's