Now I've seen lots of articles with other users experiencing similar problems, but none quite like this... I have the latest version of Firefox (5.0) and every time I install an addon I become unable to open new windows - including the manage addons window). At this point I need to restart in Firefox safe mode and disable the addon I've installed. I have tried all sorts of addons (around a dozen) and every single one gives the same result. Any addons that were installed prior to updating to 5.0 seem to work fine, however until today I haven't installed any for a couple of weeks, so that may not be related. I have checked that all addons are up to date but still no joy!