work with more URL formats
The extension currently expects the post ID to be the last item in the URL, like this: https://nextdoor.com/p/6DWSwgLL2pJM?view=detail
However, this isn't always the case. A link to a specific comment in that same thread looks like this: https://nextdoor.com/p/6DWSwgLL2pJM/c/659086436?utm_source=share
Tentatively: if the domain name is immediately followed by "/p/", the next folder-name can be assumed to be the post ID. I think there are other URL formats, and there may even be another post-ID format (all numerics), but I will document such as I come across them.
There are also some circumstances where the extension icon does not appear at all; this is, in a way, actually a less-severe issue, since it's a clear signal that the extension isn't recognizing the page as something archiveable (which is easily remedied by finding the canonical URL and loading that).
2021-09-23 Here's another URL format: https://nextdoor.com/news_feed/?post=197256351 (not the same post) -- the extension icon doesn't appear, which I assume means the extension doesn't recognize that URL as saveable.