This error message may be caused by having the Firefox cache disabled. To re-enable:
Firefox 1.5: Tools -> Options -> Privacy -> Cache Firefox 2.0: Tools -> Options -> Advanced -> Network
Make sure the cache is at least 1 MB (50 MB is the default setting).
Firewalls or spyware screeners may also cause this issue. If you use the CA firewall, Windows Defender, or the resident option of Spybot S&D ("Teatimer" feature), temporarily disable it to see if your system will now allow the extensions or themes to install [3].
Failing that, a workaround is to right-click the download link, select "Save Link As..." and download the .xpi or .jar file to your computer. You can then manually install it by dragging the file icon onto an open Firefox window [4].
Note : The Download Error 228 message may also be seen if the required .xpi or .jar file is not actually present on the server site. You can simply try again at a later time or check other download links on both the same site and a different site, before assuming that your own setup is at fault. [5]
http://kb.mozillazine.org/Unab...themes_or_extensions_-_Firefox
Greasemonkey za mozilu
koristim mozilu firefox, i na njoj bi trebalo da kao podmazano radi greasemonkey add on. medjutim kod mene (i koliko cujem - jedino kod mene 
) nece da se instalira. instalirala sam i javu, i skinula njen najnoviji update - ali i dalje nece. clenovani cookies i sve drugo moguce - ali i dalje nece. a kad pokusam da ga instaliram prijavi mi sledece:
