Gmail provides two GUI layouts: modern and classic. It may check User Agent string passed by the browser to decide which GUI will work better. For old browsers GMail may automatically choose classic layout, but you should be able to change it somewhere in Gmail settings. Your choice may be then saved in cookies and when Panda deletes cookies or blocks some advanced features (javascript, fonts download etc.), you will see classic layout again.
For newest browsers Gmail automatically uses modern layout - so if you want to see this layout by default, you should change User Agent string in your browser.
Open a new tab, go to address about:config, accept warning, and on the settings list right click a free space to add a new string named
general.useragent.override
with value (for Firefox 60 in Windows 7)
Mozilla/5.0 (Windows NT 6.1; rv:60.0) Gecko/20100101 Firefox/60.0
Use Google to search for more details, if needed.
If you're afraid you will spoil something, create a new Firefox profile and use this profile for tests. Use Google to find how to create a new Firefox profile.
Still I wonder why you stay with Firefox 47. You can turn off signature checking in Firefox 52.9.0esr and use both classic and web extensions, also unsigned or manually edited (with invalid signature).