I installed this add-on in order to know when the certificate of a website is nearly ended. I've a remark: with let's encrypt decreasing the length of their certificates, the colour-changing padlock is often red whereas this is a 'normal' situation. I would be pleased if the colors could also depends on the ratio of the certificate period. Something like:
certDays = Math.floor( (cert.validity.end - cert.validity.start) / factor )
color: diffDays < redline && diffDays < certDays/4 ? ... : ...

Useful tool. Thanks.

Great for being alert to SSL certificates that are about to expire.

Note that the colour-changing padlock is a new button (it doesn't change the browser's existing padlock), and that in 2023-era Firefox you have to add this yourself. Firefox doesn't show it among the ‘Customise Toolbar’ buttons; instead, you now need to go to the ‘jigsaw’ button which lists extensions, then click on the gear for ‘Certificate expiration’ and choose ‘Pin to Toolbar’. You can then use ‘Customise Toolbar’ to drag it to where you want it.

And also note that Mozilla blocks extensions on this Add-Ons site, so this isn't a good page for trying it out and seeing how it works!