How do Microsoft, Google and Mozilla profit from their browsers?

They profit through advertisements.

At Google, much of the profit is from they're primary products:

 

1. search

2. social

3. ads

 

This products relies on large-scale data mining.

Data mining systems are placed at the core of interactive services in a rapidly changing and sometimes adversarial environment, statistical models need to be combined with ideas from control and game theory for the best output. 

 

Google Full year financial tables with quarterly data

https://investor.google.com/financial/tables.html

 

Through agreements and declarations and private partnerships

Collecting marketing information.

the profit comes  through advertisements on them .

Advertisements with an expanded database and social contacts at large.

By selling aditional soft wares and apps as well as through advertisement of products.

they profit from their browsers from advertising shows to their users

By the speed of the response to the demand, advertising

The search engines capture the cookies and use the browsing history to display advertisement that would cater to the interests of the browsing person and thus provide an opportunity to the seller to sell their goods. This in turn brings more revenue to the browser companies.

They profit with SEO, Advertising and Social Media