Migrate Firefox plugin to WebExtensions

[Daniel Zoller]

reported by Anonymous (see https://bitbucket.org/bibsonomy/bibsonomy/issues/2781/bibsonomy-buttons-firefox-addon-needs-to)

Firefox is discontinuing support non-WebExtension addons -- the plan is for them to no longer load as of Firefox 57. According to https://compatibility-lookup.services.mozilla.com/ , BibSonomy Buttons appears to be affected. In addition, even before non-WebExtension addons become unusable, usage of BibSonomy Buttons on Firefox version 54 disables Multiprocess functionality. These links may be useful (i have no affiliation with Mozilla, i'm just a happy Bibsonomy user): https://developer.mozilla.org/en-US/Add-ons/Working_with_multiprocess_Firefox https://developer.mozilla.org/en-US/Add-ons/WebExtensions/Comparison_with_XUL_XPCOM_extensions https://wiki.mozilla.org/Add-ons/developer/communication#Don.27t_See_the_WebExtensions_APIs_You_Need.3F

[Daniel Zoller]

Removing component: Firefox (automated comment)

[Daniel Zoller]

thx!