Web services in Firefox are those localizable, functional areas within the browser, specifically, Search Plug-ins and Content Handlers (RSS Readers, Web Mail and Web Calendar). You can read more about web services at L10n:Productization.
We believe that localization teams are in the best position to provide recommendations on what local providers we can use for Web Services because you're in the market, work in the language, and know your users. This is, therefore, meant to be a guideline for how you should be formulating your recommendations.
There are typically six search plug-ins listed:
The default option should expose the quickest path to the best result on the world wide web for the user (indexing a large portion of the global www).
The second position can be used for the next best local language provider, meaning coverage within the local language and/or locale (indexing a smaller portion of the www but producing better local language results).
A good test is if you were to run a set of comparative queries; the secondary engine should produce the best local language results and the primary/default position should produce the best global results.
These search engines should allow users to get connected to new and used products they want to buy as quickly and painlessly as possible in a "trusted" site. For example choosing the most popular auction, general shopping, or classified type sites that have the desired user experience are good choices. There should be no more than two.
Specific interest search
Where user are using popular search engine's to search more specific areas of interest that are very popular in your language or locale. There should be no more than two.
In all cases, user experience should be taken into account and given preference for what is desired in that language/locale. If you have questions about what that means, please contact email@example.com or Milos on IRC.
RSS Readers and Content handlers
These should follow these guidelines: be translated in your language (because it makes the application unusable if it's not in your language), provide for the intended user experience (e.g., minimal to no advertisements within the user experience), and no more than 3 (if possible) choices. Choosing based on popularity is fine.
Some examples from en-US
|Global search||Best user experience and broadest results|
|e-Commerce Search||Amazon||Broad set of products, simple search interface with no/limited advertising, popular shopping engine|
|Specific interest||Wikipedia||Good user experience (no ads, simple interface), popular site|
|RSS Reader||Google Reader||Popular, no advertising, simple interface|