Multiproxy-Switch is a pretty nifty Firefox addon for quickly changing web proxies.
With the release of each new Firefox version, many older add-ons get culled from the typical profile due to incompatibilities. Multiproxy-Switch is no exception, and no longer seems to be updated.
I've taken this add-on on, modified it to be compatible with Firefox 4.0, and will now maintain it on github.
You can download the latest version from github.
EDIT 22/05/2011: Updated to provide Firefox 5.0 support.
EDIT 16/08/2011: Updated to provide Firefox 6.0 support.
EDIT 26/10/2011: Updated to provide Firefox 7.0 support.
EDIT 08/11/2011: Updated to provide Firefox 8.0 support, and add auto-update support.
EDIT 20/12/2011: Updated to provide Firefox 9.0 support.
EDIT 19/04/2012: No update required. Firefox 10 onwards now mark add-ons as compatible by default.
EDIT 07/05/2012: Bug fix release, updated to explicitly add support for Firefox 12.0