When using the Mozella Firefox 4 browser and the NEW version Adobe Reader X, PDF's online will not ever open. After 1-3 minuites, I eventually get a message the Adobe reader is running, but need to close and try again! I get same results when trying again. Internet Explorer operates and opens the online PDF's all ok and the Adobe Reader X seems to operate all else ok!
I think, but not sure, that this issue appeared when the Adobe Reader updated from the older version 9 or so to the NEW Adobe Reader X version about 1 or 2 weeks ago, but not sure. I just remember that Mozella always worked ok previously to the Adobe Reader X version for opening online PDF's last month, etc. Note that I said that Internet Explorer browser operates ok Using the New automatically upgraded Adobe Reader X for viewing PDF"S! (adobe notified me that the new version 10 (X) was available for updating, etc) I also just discovered that google chrome browser opens the online PDF's ok!
I have not trying re-storing the computer to the previous month, etc, since the issue seems to be with Mozella not liking the Adobe Reader X version and Internet Explored browser operating ok!
I got this problem too. I did some digging and it turns out is a known issue with Reader X. (There are issues with IE, Chrome etc etc too, just different ones.) The Adobe support website (http://kb2.adobe.com/cps/877/cpsid_87775.html) says: You can't view PDF in Firefox when both Reader 8 and Acrobat X Pro are installed. This problem occurs when Firefox was installed last. The workaround is to repair Acrobat X (Help > Repair Acrobat Installation). 
I repaired the Reader X installation as it recommended, rebooted the machine and it seems fine now.
Your hints got it running as A-OK!
In short go to mozella, tools, options, general tab, manage add ons, plugins, and choose ENABLE acrobat or reader! (the plug-in was disabled
I did not have to re-boot machine!
The solutions in the earlier posts do not work. As of this date (April 3, 2011), Firefox 4 is not compatible with Reader X. You need to uninstall Reader X and install Reader 9. The reason is that Reader X was released before Firefox 4 was officially released. There will be an update of Reader X to make it compatible, but it is not available as of today.
Yes, the above solutions worked on my computer as A-ok!
I had the same problem and my solution was to disable the PDF extension that came with Firefox 4 which belongs to another company. I found out that it wast interfering with ReaderX. I unabled it and restarted Firefox. I have had no problems since then. Go to Tools, Add-On and click on Extensions and disable the PDF extension. I hope this helps
Didn't work, I already tried this.
The above solutions/cls7272 recommended got mine to running A-Ok!
Took some time and go backs too go through the links for a fix!
I installed ADOBE ACROBAT X PRO 10.1.2 on April 8, 2012, and now I get one of two error messages when attempting to open a PDF file in either Firefox 11.0 or 12.0 Beta 4 versions: (1) There is a problem with Adobe Acrobat/Reader; and (2) The Adobe Acrobat/Reader that is running cannot be used to view PDF files in a Web Browser. There is no problem opening PDF files in Internet Explorer 9.0.5. I deinstalled Adobe Reader 9.0 thinking that might be the conflict but the same error messages appear using Firefox. Any suggestions?