Does the problem occur at the point that you expect a dialog to open to select files, or during the upload process, etc.? You might already have tried these things, but just in case: (1) To rule out one of your extensions as the culprit, can you reproduce the problem in Firefox's Safe Mode? That's a standard diagnostic tool to bypass interference by extensions (and some custom settings). More info: [[Troubleshoot Firefox issues using Safe Mode]]. You can restart Firefox in Safe Mode using Help > Restart with Add-ons Disabled ''(Flash and other plugins still run)'' In the dialog, click "Start in Safe Mode" (''not'' Reset) Any difference? (2) If Firefox lags on multiple sites (not responding, slow scripts), try disabling the protected mode feature of the Flash Player plugin. The following pages provide information on how to disable the feature. * Adobe support article under the heading "Last Resort": [http://forums.adobe.com/message/4468493#TemporaryWorkaround Adobe Forums: How do I troubleshoot Flash Player's protected mode for Firefox?] * Manual steps: https://support.mozilla.org/questions/968190?page=5#answer-509209 * Batch file to automate the manual steps: https://support.mozilla.org/questions/982093#answer-518078 That takes effect after Flash is completely unloaded; you might need to exit and restart Firefox.