You can configure Firefox to always ask which application to use when opening such content. If the application you want to use has the needed plugin already installed, it should be listed as one of the options. If not, you can usually "Browse" to the executable file of the app you want to use, usually located at /usr/bin. Here's how to make sure Firefox prompts to ask which application you want to use:
1.) open Firefox and go to Edit > Preferences
2.) select the "Applications" section
3.) find the "AVI video" listing
4.) under the "Action" heading, select "Always ask"