BUT: It does NOT work with RC-beta, no matter which OS and which browser (so not even with Win7 and FF 27):
Roundcube 1.1-git includes new multi-folder search functionality. To achieve this the way that they message list works was changed and so plugins which us it had to be changed to. There are new functions needed to find the message UID now. The git-master version of the contextmenu plugin uses these new functions, obviously the 2.0 release cannot since they do not exist in Roundcube 1.0. So for the two different versions of Roundcube you need different versions of the plugin.
Another issue: The version number of contexmenu isn't shown unter Settings/About - apparently because there's no package.xml which does contain the version number...? How to solve this?
At the moment the version information needed for the About screen is added when you download via the composer system (plugins.roundcube.net). The composer system has a few good advantages like making updates easier and making sure you get the right version of a plugin for your version of Roundcube ... ;). BTW it is my understanding that having both a composer.json file and a package.xml file is wrong. The package.xml file will never be used and plugin devs can add version information into their composer.json file if they want.