Certain Firefox problems can be solved by performing a Clean reinstall. This means you remove Firefox program files and then reinstall Firefox. Please follow these steps:

1. Download the latest Desktop version of Firefox from http://www.mozilla.org and save the setup file to your computer.
2. After the download finishes, close all Firefox windows (click Exit from the Firefox or File menu).
3. Delete the Firefox installation folder, which is located by default in:
   • C:\Program Files (x86)\Mozilla Firefox
4. Now, go ahead and reinstall Firefox:
   1. Double-click the downloaded installation file and go through the steps of the installation wizard.
   2. Once the wizard is finished, choose to directly open Firefox after clicking the Finish button.

Please report back to see if this helped you!

I did exactly what you describe , no improvement. I was surprised.

can you get firefox to run without the error message by launching it into safemode (press the shift key while the browser is starting)?

Diagnose Firefox issues using Troubleshoot Mode

I tried starting in safe mode..same problem. The error message appears, I click OK and Firefox starts fine. But it won't start without first showing the error. Curiously, I have a 2nd PC and it also updated to Firefox 22. It doesn't have this problem. I notice in the 2nd PC that the plc4.dll, etc files aren't even present in the Firefox directory. I have even copied these files to windows, system32 to see if that would help, but no luck.

hmm, can you try right-clicking the firefox setup file & run it as administrator again?

Try to reboot the computer after deleting the Firefox program folder.

Well, last night I ran Spybot instead of Malwarebytes. Found junk, removed it. Now, it seems I get the plc4 error less often but it is still there. I tried again starting in SafeMode. Apparently, I didn't have the shift key down well. Firefox starts fine, time after time, in Safe Mode. Hence, it could be a plugin or extension or theme. First I turned off Hardware Acceleration-no luck (problem persists outside of SafeMode). Next I checked the up-to-date option of my plugins and found 4 (Bing,IETab,Foxit Reader, and Amazon MP3 downloader) that were not necessarily up-to-date. I disabled each individually, restarted but the Plc4 error was still there. I disabled all 4 at once (no help). Next, I use FXchrome theme. I disabled it and used the default theme-no luck.

I am running as Administrator always on my PC.

I use all-in-one-sidebar. I notice here that hovering over each link, the one line descriptions are screwed up, while in Bookmarks each link lists correctly. I dsabled the all-in-one sidebar, restarted - no luck.

I also disabled betterfox, memory restart- no luck. I can try others later.

Any further comments? In summary may have some improvement but not resolved.

Had the same problem on two computers at home, all i needed to do to fix it was a reboot after upgrading to FF22, after that no problems so far.

Edit, dang, the problem came back :(

Which security software (firewall, anti-virus) do you have?

I use Norton security software. This is a puzzling problem. Starting in Safe Mode I never the get the error. I have tried numerous times doing a clean install-no help. I even blew away everything but my bookmarks and I exported them to an html file and then reloaded. I have installed my extensions one at a time, etc. But the problem is still with me. Sometimes, however, the problem is intermittent which is even harder to explain. After I click OK to the error message, Firefox loads and works fine without exception using all my extensions, etc.

The problem came back after i wrote my last reply, so last evening i had this several times om both computers. Today i have not had any problems yet.

Running W7 Pro on both, with Windows Defender.

Tried running Firefox with Norton turned off-no help. Also tried copying entire Firefox directory from a 2nd computer. It ran the first instance without plc4 error, but afterwards the error is back on Firefox start. I am back to my Mozilla Firefox directory. Again, after the initial directory swicth the error is gone. I have tried to restart yet.

This is a truly strange problem. I assume it's a Firefox problem and not a Windows 7 problem since I only experience this error when I start Firefox.

At this point, I am very curious what is causing this.

The saga continues..I had an older PC with Firefox v17 on it. I copied the Firefox directory to my PC, renamed the existing troublesome Firefox directory and then ran with version 17 using my extensions, bookmarks, etc. The only feature incompatible was the theme. I let Firefox update V17 to version V22. After a reboot, I thought the plc4 error was gone..but no luck. Now when I start Firefox V22 after clearing the cache, I still get the plc4 error. However once in Firefox, restarting without clearing the cache does NOT appear to give the error.

Anyone out there have any ideas.. No spyware, no viruses, etc. based upon Spybot and Norton.

I'm going get this problem yet....

The same thing is happening to me...... I have traced my plc4.dll problems to be only with Firefox 22 while using LastPass add-on. When the add-on is disabled, I do not get the error.

Everything was fine until FireFox 22.... also did a completely fresh install of FF22 to verify.

I have disabled Last Pass--so far no plc4 error. What do you use instead of Last Pass?

The problem was Last Pass..disabling it did the trick. I questioned the Last Pass support team and they tell me there is an update 2.0.26--installed and so far so good.