Some password manager services let you access your passwords through your web browser, but F-Secure does not; this is due to the security risks involved.
There are several differences between the technical options available for password managers that run in your web browser and apps that are installed separately, for example in how effectively the saved data can be encrypted and securely stored. How different web browsers transfer and store data, even temporarily, from the sites and services that you visit can also have an impact on the security of your password data.
For more in-depth examples on the risks of web-based password managers, see https://www.usenix.org/system/files/conference/usenixsecurity14/sec14-paper-li-zhiwei.pdf and https://grahamcluley.com/2015/11/flaws-lastpass-password-manager-security-researchers/.