GreaseMonkey script that allows to quickly add an email to the "from" field of an existing filter.
Notes:
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Works only when Gmail is in English (must say "edit" to edit a filter)
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Filters that should be included in the dropDownList, must include the following string (including the quotes) in the "Doesn't have" field:
"filterName:<FilterName>"<FilterName>: should be replaced by the wanted filter name, can contain spaces. E.g.: "filterName:Friends Emails"