I am unable to search for Firefox bookmarks in KRunner.
It works with Konqueror and Falkon (but I don’t want to use these browsers). I find out the browser has to be set as the default browser for the feature to work. But it doesn’t work with Firefox.
This functionality isn’t natively supported in KRunner. Konqueror and Falkon are supported because they are programs made by KDE (and so have a relatively good integration with KDE Plasma). Firefox, however, doesn’t supply the necessary things to KRunner (or KRunner can’t get the necessary things from Firefox, depending on how you want to see it).
There was (once) a plugin to get Firefox Bookmarks in KRunner, and it is still listed under System Settings > Search (in the workspaces category) > Plasma Search > Get New Plugins (top right). However, it isn’t maintained and (at least on my system) it fails to install.
If that plugin works on your system, then it’s great! If it doesn’t, that’s not so great…