Visual test with Firefox and tox (#1513)

* Visual test with Firefox and tox

Allows developer to run

```
tox -e py310-sphinx72-firefox
```

This will run the tests and open a Firefox after building the demo site
showing the `demo/demo.html` page on it.

Then, you can also run:

```
tox -e py310-sphinx61-firefox
```

and compare the visual differences.

Idea copied from #1388
Closes #1388

* Default value for `DISPLAY` and `--new-tab`

* Simplify the idea :)

tox.ini

@@ -25,6 +25,9 @@ deps =
     sphinx72: Sphinx>=7.2,<7.3
     sphinxlatest: Sphinx
     dev: https://github.com/sphinx-doc/sphinx/archive/refs/heads/master.zip
+allowlist_externals =
+    echo
 commands =
     pytest {posargs} tests/
     sphinx-build -b html -Dhtml4_writer=0 -d {envtmpdir}/doctrees docs/ {envtmpdir}/html
+    echo "Open the following URL for visual testing: file://{envtmpdir}/html/demo/demo.html"