Why????

No supported PAC script.

Latest version requests permissions to delete browsing data. Why proxy management software would do that?
Please move that function into a dedicated plugin and use only necessary permissions in each.

UPDATE:
I'm not worried about a plugin stealing my data. I just don't want to have a bloated do-it all plugin, that uses permissions it does not need.

I agree, that different contexts when using different proxies may be useful, but that is supposed to be done with https://developer.mozilla.org/en-US/Add-ons/WebExtensions/API/contextualIdentities and not with data deletion. Even then, it's probably a scope of a different dedicated plugin.

I would also like to point out, that if you decide to remove browsing data access later, it will be a removal of a well-used feature (and much more users will come yelling), while now it is an addition of an unnecessary quirk.

To be less negative, let me say it in a different way. I adore the simplicity of previous version, with a little bit of UI polishing it would easily become my favorite plugin. Please don't take that true quality of a dedicated tool from us.

Please consider reverting the new Cookie permission requirement. I prefer this add-on to be solely resposible for handling proxy connections.

Why would I want to allow a proxy switcher to clear my recent browsing history, cookies, and related data!? A proxy switcher should only make sure the appropriate proxy is used nothing else.

Please add mobile ff support.

Editing my review, around 6 years later, after an ABSOLUTELY DISASTROUS update (v8.2 it seems) that got automatically applied today (this is why auto-updates are bad and I used to disable them all the time - I should have kept this up).
I use multiple separate profiles, each with distinct proxy settings. The update lost the extension settings fully in half of my profiles. In one of them (my main one, mind you), it SILENTLY DISABLED the SOCKS proxy that I had configured via Firefox's settings.
Luckily, I checked my IP that same day for another reason so I was able to detect that without my IP getting leaked for days or weeks or more without me noticing. But this could have been much worse. Totally unacceptable.

---- Old review below:

I used that add-on a bit pre-Fx 57, and it felt quite bloated at the time.

I tried it again after the WebExtension mayhem, and I really like this new version. For my rather simple use case (being able to use and switch between a bunch of SOCKS5 proxies that require username+password), it's quite perfect. I also like that it's easy to import/export settings, so that I can duplicate my config between my several profiles.

As someone else said, it would be nice to be able to disable logging, and apparently that feature is coming soon :)