Is it permissible for an employee whose work is not connected to the interests of others to postpone tasks, browse the internet, or leave early, knowing that he sometimes exerts extra effort? And does this affect the acceptance of prayers?

Your ruling is that of a private employee. You are required to adhere to the specified working hours and not violate work regulations except with permission. It is not permissible for you to exploit work time for matters other than the company's interests. If you finish your work, there is no harm in utilizing the time for permissible activities, with the permissibility of using work tools that are not affected by use, such as using free internet. It is better to seek permission from your employer for this.

Negligence in work leads to consuming ill-gotten wealth, which is one of the impediments to the acceptance of supplication. The Messenger of Allah, peace and blessings be upon him, said: "O people, indeed Allah is good and accepts only what is good..."