In File - Wizards - Address Data Source or Tools - Address Data Source - Assign, 1st screen has "Select Other external data source", 2nd screen has just "Settings" button and 3rd screen has dropdown list, which are just 3 additional clicks. Instead, all data sources: Firefox, Thunderbird and all sources currently in dropdown list should be available in 1st screen.
Wizard in a wizard... no idea if this can be simplified so easily (of course, the Settings buttons can be placed next to "Other" on the first page).
I agree with Timur. The last option Other external should have a dropdown near with all posibilities from screen 3.
We discussed the topic in the design meeting. The intermediate steps such as the connection to a data source vary depending on the type. For a spreadsheet the location needs to be specified, in case of text files the file type, separator, names, etc. too. Plus, these dialogs are reused and quite complex. We should rather focus on a better one-click integration of TB (bug 138715) and other.