While trying to install AdBlock Plus on Firefox 3.1, I encountered an error similar to this guy:

Firefox: Firefox could not install the file at ‘URL’ because: Signing could not be verified.

I was able to follow those steps, but with a bit of variation:

1. Made a temp folder
   mkdir /tmp/adblock
2. Get the extention (I copied the URL from the failed install error message):
   wget https://addons.mozilla.org/downloads/file/74835/adblock_plus-1.1.3-fx+sm+tb+fn.xpi?src=api –no-check-certificate
3. Unzip the package:
   unzip adblock_plus-1.1.3-fx+sm+tb+fn.xpi
4. The first bit of output showed the RSA file he looks for in Step 5:
   Archive:  adblock_plus-1.1.3-fx+sm+tb+fn.xpi
   inflating: META-INF/zigbert.rsa
5. Search for the Certificate Authority info:
   mikes@mikes-pc:/tmp/adblock$ strings ./META-INF/zigbert.rsa | egrep -i “sign|cert”
   “Secure Digital Certificate Signing1806
   $StartCom Verified Certificate Member1
   “Secure Digital Certificate Signing1)0′
   StartCom Certification Authority
   Limited Liability, read the section *Legal Limitations* of the StartCom Certification Authority Policy available at http://www.startssl.com/policy.pdf0c
   4http://www.startssl.com/certs/sub.class2.code.ca.crt0#
   “Secure Digital Certificate Signing1)0′
   StartCom Certification Authority0
   “Secure Digital Certificate Signing1806
   “Secure Digital Certificate Signing1)0′
   StartCom Certification Authority
   &http://cert.startcom.org/sfsca-crl.crl0′
   #http://cert.startcom.org/policy.pdf05
   )http://cert.startcom.org/intermediate.pdf0
   AStartCom Class 2 Primary Intermediate Object Signing Certificates0
   “Secure Digital Certificate Signing1806
6. Unlike his Step 7, in Firefox / Preferences / Advanced / Encryption / View Certificates / Authorities, that cert wasn’t listed.  So I imported it.
7. Click the Import button on the Authorities tab.  Browse to the /tmp/adblock/META-INF folder created when AdBlock was unzipped.
8. Change the file type selector from Certificate Files to All Files; select zigbert.rsa; click Open.
9. Check the box, “This certificate can identify software makers.”
10. Click OK until enough dialog boxes are closed that you can re-try the AdBlock Plus install.

It then installed without issue.  Restart Firefox to enable the add-on.