notifications from firefox's "auto tab discard" plugin never disappear

the extension "auto tab discard" (https://addons.mozilla.org/en-US/firefox/addon/auto-tab-discard/) sends notification upon for example whitelisting. however, these notification never disappear, despite having set `default-timeout` in the config file.

`~/.config/mako/config`:
```
default-timeout=5000
```

and yes, firefox is communicating with mako, so it is not [this issue](https://github.com/emersion/mako/issues/126). maybe this somehow is related to the same issue that was discovered in #154?