This is a top-notch add-on and I'm so glad I found it. Here's how I use it:
Especially the fully customizable Advanced Options field is worth checking out, because this lets you append any Google search parameter to the query. Use this feature to filter results on the fly, by one or more languages (&lr=lang_xx), by country (&cr=countryXX) and even boost results from a particular country (&gl=xx). You could also filter by publication date range.
This immensely improves the precision of your queries. Any Google search parameter can be used in this way, so it's useful to keep a list of current Google web search parameters at hand.
Couple of suggestions for improvement to consider:
1. for quick access, assign hotkeys to Google Global search queries
2. add customizable date range searches, e.g. "past three months", where Google Global automtically substitutes the mdy values for startdate and today's date:
&tbs=cdr:1,cd_min:[startdate m]/[startdate d]/[startdate yyyy],cd_max:[today m]/[today d]/[today yyyy]
3. Add auto-lookup lists for Google search parameters and values, such as country codes, interface languages, search result languages etc.
The Google Global add-on works like a charm on Firefox beta v. 19.0.