What plugin are you working on? The best way is to go to the website of the program and get the full installer.

None of them work! I have tried downloading Adobe reader and Java. Same result. I also cannot browse through drop down menus. I have turned off acceleration, tried safe mode, tried refreshing and still does not work!

Sometimes the pluginreg.dat file becomes corrupted. You can remove it and have Firefox re-scan for plugins. As a side effect, any plugins you have disabled on the Add-ons page probably will be re-enabled, so you'll need to go back to that page one more time.

Type about:support in the address bar and press enter.

Under the page logo on the left side, you will see Application Basics. Under this find Profile Folder. To its right press the button Show Folder. This will open your file browser to the current Firefox profile. Now Close Firefox.

Locate the above file. Then rename or delete it. Restart Firefox.