How do I manually refresh or reload my music library?

Yup, that's the way to do it.
To start complete fresh, mark all entries in you library (ctrl + a), hit del and all tracks will vanish. Then hit up file>import media to start anew.

That's a really idiotic approach from the developers side in my humble opinion. I got over 50'000 MP3's and it takes for ever if i have to refresh my whole library every time I get a new album. There should be some feature to scan for changes in the currently existing library. But I guess the developers already working on that one. :/

Where or how do I actually type this code or change this setting?

open a new tab, enter about:config, look for the property and change the value

Does modifying this config value reset play statistics and rating stars?

I love this post. I have missed it and wrote one by myself ("refresh library: delete broken links, re-read metadata information"). In this post 2 workaround are suggested. I have tested them both on a small music DB (ca. 400 MB) and a big one (ca. 37GB)

Trick 1 (Lews))
Library --> ctrl a --> right click --> Remove ==> this empties your library
File --> Import Media --> brows to your music directory --> OK ==> this fills up your library again (scans from scratch the music directory)

Trick 2 (Ali Rayl, Official Rep))
File --> New Tab --> about:config --> change songbird.commands.enableRescanItem to true --> close songbird --> Library --> ctrl a --> right click --> Rescan

On the small DB both did work properly.
On the big DB the Trick 1 did work properly, the Trick 2 did not work (I did change metadata and names of some files and after the rescan they were still wrong in the library).

So 2 things:

1) I think this is a very very important feature: it must be officially and not via workaround supported
2) If the workaround is "more or less official" should be easier to find
3) The "official "workaround" (trick 2) does not work: please check it and fix it!

The dev trick works very nicely for me, but I only use it on a few songs at a time.

Beware, guys. The resetting of the value in about:config is just a rough, magical thing, not meant to be used by the normal users.
Use it with care and don't expect wonders though.

We are definitely working on improving media handling and scanning.
The next release, that shall come out very soon indeed has many changes in this particular field.

So hang in there with us.
Tyler out!