I'm been using Jira for nearly one year, and being too stressful that i need to submit a suggestion.
Every page has a fixed header, which takes up 1/3 of my screen.
Whenever i want to view tasks created, i have to scroll within a small table (inner scroll) and this is very stressful.

The above part of the page (1/3 of the screen) are non usable, with only 2/3 visible on the bottom, and then task lists created within that area with inner scroll within a table, which means only a few lines visible everytime. 

With at least 50tasks in a project, can you imagine the stress to scroll a few lines again and again ? (only 1/3 of the screen is viewable space)
Cant. have a full view of all the tasks.

Strongly suggest to NOT to make the header fixed, or at least you can make it smaller, a small narrower design, and not taking up nearly half of the space.

And also for task list, why is there inner scroll within a table ?
Just scroll the page, the table doesnt need to feed on the screen.
the UI is too horrible. Please kindly take a look into it.