I suggest this procedure to replace the entire program folder from a known good source:

Clean Reinstall

(1) Download a fresh installer for Firefox 34 from https://www.mozilla.org/firefox/all/ to a convenient location. (Scroll down to your preferred language.)

(2) Exit out of Firefox.

(3) Rename the program folder

(32-bit Windows folder names)

C:\Program Files\Mozilla Firefox

to

C:\Program Files\OldFirefox

(64-bit Windows folder names)

C:\Program Files (x86)\Mozilla Firefox

to

C:\Program Files (x86)\OldFirefox

(4) Run the installer you downloaded in #1. It should automatically connect to your existing settings.

Then you can start Firefox up again (and restore your earlier session using the History menu, if applicable) and test.

Any improvement?

Note: Some recently installed plugins may exist only in that OldFirefox folder. If something essential is missing, look in these folders:

• \OldFirefox\Plugins
• \OldFirefox\browser\plugins

That said, WinZip has no business using a DLL in Firefox's program folder, so there could be a problem elsewhere on the system.