Access the Context Menus with keyboard

**Steps to reproduce:**

Context menus from both the Timeline and the Details panels aren't accessible with a keyboard.

**Expected behavior:**

There should be a way of accessing the Context Menu with a screenreader/keyboard.

**Actual behavior:**

There's no way to access the context menu with a screenreader/keyboard.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-103)