1. Install Silent Block.
2. Create the file contentblock-regex.txt in your Seamonkey/Firefox profile directory and fill it with:

   ^https?://addthis.*
   ^https?://[a-zA-Z0-9]+.addthis.*
   ^https?://doubleclick.*
   ^https?://[a-zA-Z0-9]+.doubleclick.*
   ^https?://facebook.*
   ^https?://[a-zA-Z0-9]+.facebook.*
   ^https?://google.*
   ^https?://[a-zA-Z0-9]+.google*
   ^https?://twitter.*
   ^https?://[a-zA-Z0-9]+.twitter.*

Tested to work with:

• Firefox Portable 47.0
• Firefox 42.0 x86

1. Search for the plugin on https://addons.mozilla.org.
2. From Firefox:
   1. Right-click the “Add to Firefox” button.
   2. Click on “Save Link As…”.
   3. Save the .xpi file.
3. From Internet Explorer:
   1. Right-click on the “Download Anyway” link.
   2. Click on “Save target as…”
   3. Save the .xpi file.
4. Close Firefox.
5. The .xpi file is zip file. Open it in an archive manager or extract it.
6. Open the install.rdf file in/from the xpi file.
7. Locate the <em:id> </em:id> tags.
8. Copy the value between them.
9. Make a copy of the .xpi file and rename it to the value copied from between the <em:id> </em:id> tags, but maintain the .xpi file extension.
10. Place the renamed .xpi file in:
    1. Firefox: Mozilla Firefox\browser\extensions
    2. Firefox Portable:
       1. App\Firefox\browser\extensions
       2. App\Firefox64\browser\extensions
11. Start Firefox.
12. You either get:
    1. The plugin visible under addons, but disabled. Tested on Firefox 42.0 x86.
    2. A question from Firefox to activate the plugin. Tested on Firefox Portable 47.0.

Sources:

• MozillaZine KB - Installing extensions
• MozillaZine KB - Determine extension ID
• MDN - Installing extensions