Update our browserslist. (#24919)
Update our browserslist. (#24919)

Update our browserslist.

* Add `last 1 major version` and `>= 1%`.
* Add Firefox.

Firefox v38 is older than the ESR, so basically replace ESR with v38.

seanwu99/bootstrap,creativewebjp/bootstrap,gijsbotje/bootstrap,tagliala/bootstrap,gijsbotje/bootstrap,inway/bootstrap,Lyricalz/bootstrap,bardiharborow/bootstrap,twbs/bootstrap,janseliger/bootstrap,stanwmusic/bootstrap,seanwu99/bootstrap,kvlsrg/bootstrap,joblocal/bootstrap,yuyokk/bootstrap,joblocal/bootstrap,bardiharborow/bootstrap,Hemphill/bootstrap-docs,twbs/bootstrap,seanwu99/bootstrap,joblocal/bootstrap,GerHobbelt/bootstrap,SweetProcess/bootstrap,creativewebjp/bootstrap,yuyokk/bootstrap,fschumann1211/bootstrap,Hemphill/bootstrap-docs,Hemphill/bootstrap-docs,peterblazejewicz/bootstrap,stanwmusic/bootstrap,inway/bootstrap,Lyricalz/bootstrap,tjkohli/bootstrap,inway/bootstrap,m5o/bootstrap,kvlsrg/bootstrap,fschumann1211/bootstrap,seanwu99/bootstrap,bootstrapbrasil/bootstrap,creativewebjp/bootstrap,SweetProcess/bootstrap,nice-fungal/bootstrap,supergibbs/bootstrap,yuyokk/bootstrap,peterblazejewicz/bootstrap,supergibbs/bootstrap,stanwmusic/bootstrap,supergibbs/bootstrap,yuyokk/bootstrap,tagliala/bootstrap,vsn4ik/bootstrap,coliff/bootstrap,zalog/bootstrap,SweetProcess/bootstrap,supergibbs/bootstrap,Lyricalz/bootstrap,tagliala/bootstrap,fschumann1211/bootstrap,stanwmusic/bootstrap,SweetProcess/bootstrap,bootstrapbrasil/bootstrap,peterblazejewicz/bootstrap,janseliger/bootstrap,9-chirno/chirno,joblocal/bootstrap,creativewebjp/bootstrap,coliff/bootstrap,m5o/bootstrap,zalog/bootstrap,vsn4ik/bootstrap,vsn4ik/bootstrap,gijsbotje/bootstrap,janseliger/bootstrap,nice-fungal/bootstrap,Lyricalz/bootstrap,GerHobbelt/bootstrap,vsn4ik/bootstrap,bootstrapbrasil/bootstrap,gijsbotje/bootstrap,bardiharborow/bootstrap,Hemphill/bootstrap-docs,GerHobbelt/bootstrap,zalog/bootstrap,9-chirno/chirno,fschumann1211/bootstrap,janseliger/bootstrap,tjkohli/bootstrap,kvlsrg/bootstrap,9-chirno/chirno,bardiharborow/bootstrap,9-chirno/chirno,nice-fungal/bootstrap