I'm getting this error when I try to upgrade Flash Player for Firefox and IE:
Only a single instance of this application can run..
Any idea how to update Flash player then?
I think this problem is somewhat recent. I had a similar issue. My solution was:
It's a bit annoying because between steps 1 and 2 you need to reboot. But it worked for me.
The upgrade for Flash player for IE and Firefox is two different programs. For IE, it is an ActiveX control, for Firefox (and the other browsers), it is a plugin. It looks like you were somehow running both simultaneously.
Go to this site with BOTH browsers and check your plugins:
http://www.mozilla.com/en-US/plugincheck/ (it says Mozilla, think Firefox, but works for any browser)
I suspect you may have actually already updated one (the first one that was running), and that only the second failed. Any plugins not updated will be reported to you, and will link you to the download. I might also suggest that you reboot and try this. Do one browser at a time. Give this a try, and if you actually have more serious problems, post back.
I have been fighting this problem for over a week now. Just figured things out today. I had installed Microsoft Silverlight a bit ago. I think that is the "other instance" that the computer was trying to tell me was interfering with installing Adobe Flash. I think it is a version of Flash Player. I uninstalled it and then installed the 64 bit version of Adobe Flash for other browsers (other than Iinternet Explorer). The Flash player now works with Safari and Firefox. There was a suggestion to download Google Chrome since it has an integrated Flash player and I did that. It worked great from the start, but I still wanted to use my other browsers. There was a version of Flash Player just for Internet Explorer, but I did not install that since I am uncertain whether or not I am going to use Explorer going forward. It has been acting up. I really hopes this helps some of the rest of you.