I added a shortcut key to start my browser(Ctrl+F) but later found it was used by another program.
Is there a way to get a list of shortcuts already in use?
Firefox is my browser.
It is not possible to have one comprehensive list of keyboard shortcuts that are in use. That is because keyboard shortcuts may be defined and act on different levels.
Keyboard shortcuts used in an application are defined by the developers of the application. In some cases, the application allows to reconfigure these definitions, in other cases, it doesn't.
Keyboard combinations can defined by your desktop environment. These are active throughout and have preference over the shortcut defined in an application. For example, if you define Ctrl+f to launch your browser, then the same shortcut defined at the level of an application, i.e. Ctrl+f in your browser or word processor to find something, will not work anymore.
For (1), there may be a list of the shortcuts in the manual, or on a help page, or there might be no documentation. It depends on the software. For (2), you can typically see these in the Keyboard settings of your desktop environment (Gnome, Mate, Budgie, KDE, ...), or in the configuration manual of window managers (openbox, i3wm, ...).
There are some conventions on the use of certain keyboard combinations. Typically, programmers will attempt to define their hotkeys as to not conflict with combinations that find general use (otherwise they will hear it from the users!). You thus need to have the same care in choosing your hotkeys. If at some point, you discover that your choice of Ctrl+f to open a browser conflicts with an important hotkey in an application, then you will need to consider whether to keep that combination or not.