Who became the first feminists when they realized that in many ways they were slaves also?
This is actually a controversial question.
Feminism is said to have started with First Wave feminism of the 19th century when [white] women chose to fight for the right to vote, own land, and equal right in marriage. The controversial part comes in that they fought purely for the rights of white women, overwhelmingly feminism has remained about white women and often excluded women of colour - for example, slaves.
Search for the term: 'White Feminism' and #solidarityisforwhitewomen