ClearURLs toolbar button icon: the filter sometimes appears to be on (amber), when off (should be grey)

First observed with 1.3.3.16 with Waterfox 56.2.3 on FreeBSD-CURRENT. Reproduced with Firefox 62.0_2,1, with a new profile.

Steps

1. Add the extension
2. switch off Filter
3. observe the change of colour of the icon – from amber, to grey
4. quit
5. start the browser

Expected

• grey

Actual result

• amber

… and it took a few weeks for me to realise that filtering was off.