Navigation

Back - Alt + Right_Arrow_Key or Ctrl + [
Forward - Alt + Left_Arrow_Key or Ctrl + ]
Reload - Ctrl + R or F5
Stop Reloading - Esc
Force Reload - Ctrl + Shift + R or Ctrl + F5
Open File - Ctrl + O
Home - Alt + Home

Current Page

Focus Next Interactive Element - Tab
Focus Previous Interactive Element - Shift + Tab
Scroll One Page Down - Page_Down
Scroll One Page Up - Page_Up
Bottom of Page - End or Ctrl + Up_Arrow_Key
Top of Page - Home or Ctrl + Down_Arrow_Key
Open Print Menu for current page - Ctrl + P
Zoom In - Ctrl + Plus
Zoom Out - Ctrl + Minus
Zoom Reset - Ctrl + 0
Bookmark - Ctrl + D

Searching

Find - Ctrl + F
Find Next - Ctrl + G or F3
Find Previous - Ctrl + Shift + G or Shift + F3
Exit - Esc

Windows

Close Window - Ctrl + Shift + W or Alt + F4
Quit Window - Ctrl + Q
New Window - Ctrl + N
New Private Window - Ctrl + Shift + N
Undo Close Widnow - Ctrl + Shift + N

Tabs

Close Tab - Ctrl + W or Ctrl + F4
Cycle Tabs - Ctrl + Tab * Need to enable in settings
Move to left tab - Ctrl + Page_Up
Move to right tab - Ctrl + Page_Down
Switch to tab 1-8 - Ctrl + 1 to 8
Switch to last tab - Ctrl + 9
Toggle Mute for current Tab - Ctrl + M
New Tab - Ctrl + T
Undo Close Tab - Ctrl + Shift + T

Miscellaneous

History - Ctrl + H
Bookmarks - Ctrl + B
Developer Tools - Ctrl + Shift + I
Toggle Fullscreen - F11
Toggle Reader Mode - Ctrl + Alt + R

Reference: https://support.mozilla.org/en-US/kb/keyboard-shortcuts-perform-firefox-tasks-quickly