Is the feature relevant to the Firefox PDF Viewer?

Yes

Feature description

Pressing 'ESC to deselect the current annotation tool would be a good addition and I reckon that should be relatively easy to apply since shortcuts like CTRL+Z are already in effect to revert an edit.

This would improve accessibility, especially for laptop users who rely on their trackpads and keyboard shortcuts.

Other PDF viewers

Yes, it's already present in both MS Edge's native viewer and the new Adobe powered one.