You can do this by changing 2 hidden preferences.

1. Type about:config into the location bar and press enter
2. Accept the warning message that appears, you will be taken to a list of preferences
3. In the filter box type view_source
4. Double-click on view_source.editor.external to change its value to true
5. Double-click on view_source.editor.path and set it to C:\Program Files\Notepad++\notepad++.exe (if Notepad++ is stored elsewhere change that to where it is stored)

You can also do this by using the Dafizilla ViewSourceWith add-on.

You have out-of-date plugins. It is important to keep them updated due to continuing security fixes and improvements in those plug-ins:

• Adobe PDF Plug-In For Firefox and Netscape
• Java Plug-in 1.6.0_12 for Netscape Navigator (DLL Helper)
  • Firefox 3.6 needs Java 1.6.0_15 or higher (see: http://support.mozilla.com/en-US/kb/Using+the+Java+plugin+with+Firefox)

1. Check your plugin versions: http://www.mozilla.com/en-US/plugincheck/
   • Note: plugin check page does not have information on all plugin versions
2. Update Adobe Reader (PDF plugin):
   • From within your existing Adobe Reader (if you have it already installed):
     • Open the Adobe Reader program from your Programs list
     • Click Help > Check for Updates
     • Follow the prompts for updating
     • If this method works for you, skip the "Download complete installer" section below and proceed to "After the installation" below
   • Download complete installer (if you do NOT have Adobe Reader installed):
     • Use Firefox to download and SAVE the installer to your hard drive from the link in the article below
     • On the Adobe page, un-check any extra items (i.e. Free McAfee® Security Scan Plus) then click Download button
     • If you see message just under tabs, DO NOT click the Allow button, instead click below that where it says "click here to download".
     • Click "Save to File"; save to your Desktop (so you can find it)
     • After download completes, close Firefox
     • Click the installer you just downloaded and allow the install to continue
       • Note: Vista and Win7 users may need to right-click the installer and choose "Run as Administrator"
     • Download link and more information: https://support.mozilla.com/en-US/kb/Using+the+Adobe+Reader+plugin+with+Firefox#Installing_and_updating_Adobe_Reader
   • After the installation, start Firefox and check your version again.
3. Update Java:
   • Download and update instructions: https://support.mozilla.com/en-US/kb/Using+the+Java+plugin+with+Firefox
   • Removing old versions (if needed): http://www.java.com/en/download/faq/remove_olderversions.xml
   • After the installation, start Firefox and check your version again.

Thanks, that worked great :)

worked great, but only for whole page. how make it for selected text?

Followed the steps, and restarted computer -- NOT WORKING -- View Page Source still uses Firefox's default viewer.

Rigorous details of what I did (probably no surprises here): (1) I typed "about:config" in the Firefox address box, and hit enter; I typed "view_source" in the Filter box, and hit enter. (2) I double-clicked view_source.editor.path; I was prompted to input the path into a text box. (3) I right-clicked the Notepad++ icon, and selected Properties. The Target: text box showed "C:\Program Files\Notepad++\notepad++.exe" highlighed. (4) I copy/pasted into Firefox's text box for view_source.editor.path, and clicked Ok. it now shows "C:\Program Files\Notepad++\notepad++.exe" (quote marks included). (5) I double-clicked view_source.editor.external, so that its value now shows as "true". (6) I closed all Firefox windows, and even restarted the computer.

Try getting rid of the quotes.

Also test the path to the exe in Start | Run.