This is a known problem with the protected mode feature of the Flash player plugin on some systems, While that feature has security benefits, not everyone is able to use it. There is a hidden setting to disable it:

(A) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(B) In the search box above the list, type or paste flash and pause while the list is filtered

(C) Double-click the dom.ipc.plugins.flash.disable-protected-mode preference to switch its value from false to true.

This might not take effect until all Flash has been unloaded for a few minutes, or you close Firefox.

Could you try that for a while and see whether it fixes the problem?

You know, that hidden setting might not exist in Firefox 34. It might have been added in Firefox 36. Before it was added into Firefox, you had to do it this way:

You need to create or edit an Adobe settings file. The following pages/posts provide different approaches for that if it turns out to be necessary (if Flash EXE processes continue to show up in the Windows Task Manager, Processes tab, even after changing the Firefox preference):

• Adobe support article under the heading "Last Resort": 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 (alternate version of Carm's batch file with a few changes by me: https://onedrive.live.com/?cid=f7d304d92388737d&id=F7D304D9238...)

Flash needs to completely unload from memory (exiting and starting Firefox up again might help) before this takes effect.