Chrome

• Press CTRL-SHIFT-I
• Then, Press CTRL-SHIFT-P
• Start typing 'Disable JavaScript' (without the quotes / can be lowercase)
• Click on the highlighted line to disable JavaScript for that tab only

Firefox

• Open Dev tools with CTRL-SHIFT-I
• Click on Toolbox Options (the cog)
• Select 'Disable JavaScript *' to disable JavaScript in Firefox for that tab only

Note: In both Chrome and Firefox, this state is cleared once the tab is closed.