Nimbus Screen Capture is a lightweight Firefox add-on that can directly capture entire webpages, selected regions and active window.
Download and install it for Firefox from here.
Once installed, it will show up next to address bar.
There are different ways to take screen captures using Nimbus : either click on the add-on icon and then choose what needs to be captured, or use the shortcut keys.
To configure these shortcut keys, choose Options from the add-on menu and assign the shortcuts as needed.
It also has a screenshot editor. The captured image can be edited to add annotations, symbols, text and so on.
Once the screenshot is captured and edited, it can be saved locally, to Nimbus, Google drive or be printed. Comments for these saved screenshots can be added for future reference.
There are other add-ons too like Easy Screenshot for Firefox. For using a local desktop tool for this, Greenshot is quite useful. Also to automate screenshots and schedule them in Windows, Auto Screen Capture does the job.