The same as with Google's previous deals with Mozilla, Opera, etc.
By being the default search engine, Google gets more prominent placement in the browser. Were Google to no longer be the default in Firefox, some users would use alternatives to Google - either because they just leave the non-Google default because it is good enough, or because they are exposed for the first time to the alternatives, and they find they like them - all of which Google wants to prevent at all costs.
Another way to look at this is that there were some estimates thrown around during the Flock days of how much advertising revenue a browser generates for a search engine (this was Flock's proposed business model). I think I remember $1-2 per user per year. That should actually be higher today, assuming in the ensuing years search engines have gotten better at monetizing traffic which seems reasonable. Given that, $300M sounds about right given the amount of users Firefox has - apparently Firefox was receiving below market value before.
The same as with Google's previous deals with Mozilla, Opera, etc.
By being the default search engine, Google gets more prominent placement in the browser. Were Google to no longer be the default in Firefox, some users would use alternatives to Google - either because they just leave the non-Google default because it is good enough, or because they are exposed for the first time to the alternatives, and they find they like them - all of which Google wants to prevent at all costs.
Another way to look at this is that there were some estimates thrown around during the Flock days of how much advertising revenue a browser generates for a search engine (this was Flock's proposed business model). I think I remember $1-2 per user per year. That should actually be higher today, assuming in the ensuing years search engines have gotten better at monetizing traffic which seems reasonable. Given that, $300M sounds about right given the amount of users Firefox has - apparently Firefox was receiving below market value before.