Issues with keyboard access of the context menu

The menu is available through keyboard shortcuts on the hidden thread indicator.

STR1:

1. display the menu using the keyboard
1. without moving the cursor, press "enter" again
=> the first element is checked/unchecked, despite that it's not selected

STR2:

1. display the menu using the keyboard
1. move the cursor on a line that's not the first one (that makes the issue more visible)
1. press enter
=> notice how it looks like that the menu is "reloaded", especially we lose the selection position.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-131)