Problems

Elements (via xpath, id or name) are not found with the Firefox driver. Chromium works fine, too.

Reason is that the webdriver spec changed lately, the "web element identifier" is now element-6066-11e4-a52e-4f735466cecf instead of the previous ELEMENT. geckodriver already follows this, chromedriver not.

https://github.com/instaclick/php-webdriver/commit/be87042314cc654f7ff3e26c20a998cadb23daac

WebDriver with Firefox

https://w3c.github.io/webdriver/webdriver-spec.html

1. Run your local firefox installation in marionette mode:

   $  firefox --marionette
   1508154717660       Marionette      INFO    Enabled via --marionette
   1508154718454       Marionette      INFO    Listening on port 2828
   

2. Run geckodriver WebDriver proxy:

   $ ./bin/geckodriver --connect-existing --marionette-port 2828
   1508154721745       geckodriver     INFO    geckodriver 0.19.0
   1508154721755       geckodriver     INFO    Listening on 127.0.0.1:4444
   

3. Use curl to control it:

   $ curl -s -d '{}' http://localhost:4444/session | jq .
   {
     "value": {
           "sessionId": "f1c4ec02-3edc-4308-a0a8-697edd383b9a",
           "capabilities": {
             "acceptInsecureCerts": false,
             "browserName": "firefox",
             "browserVersion": "56.0",
             "moz:accessibilityChecks": false,
             "moz:headless": false,
             "moz:processID": 26238,
             "moz:profile": "/home/cweiske/.mozilla/firefox/u05dh3q4.default",
             "pageLoadStrategy": "normal",
             "platformName": "linux",
             "platformVersion": "4.12.3cw",
             "rotatable": false,
             "specificationLevel": 0,
             "timeouts": {
               "implicit": 0,
               "pageLoad": 300000,
               "script": 30000
             }
           }
     }
   }
   
   $ session=f1c4ec02-3edc-4308-a0a8-697edd383b9a
   
   $ curl -s -d '{"url":"http://example.org"}' http://localhost:4444/session/$session/url | jq .
   {
     "value": {}
   }
   
   $ curl -s  http://localhost:4444/session/$session/title | jq .
   {
     "value": "Example Domain"
   }
   
   $ curl -s  http://localhost:4444/session/$session/url | jq .
   {
     "value": "http://example.org/"
   }
   
   $ curl -s -d '{"using":"xpath","value":"//h1"}'  http://localhost:4444/session/$session/element | jq .
   {
     "value": {
       "element-6066-11e4-a52e-4f735466cecf": "4131ac8d-651c-4903-8b04-a591f4f7456f"
     }
   }
   
   $ curl -s -d '{"using":"xpath","value":"//h1"}'  http://localhost:4444/session/$session/elements | jq .
   {
     "value": [
       {
         "element-6066-11e4-a52e-4f735466cecf": "30f5c6ca-6fc5-4f23-b331-d102872b4238"
       }
     ]
   }
   
   $ curl -s -d '{"using":"xpath","value":"//doesnotexist"}'  http://localhost:4444/session/$session/element | jq .
   {
     "value": {
       "error": "no such element",
       "message": "Unable to locate element: //doesnotexist",
       "stacktrace": "stack backtrace:\n   0:           0x4f388c - backtrace::backtrace::trace::h736111741fa0878e\n   1:           0x4f38c2 - backtrace::capture::Backtrace::new::h63b8a5c0787510c9\n   2:           0x442d88 - webdriver::error::WebDriverError::new::hea6d4dbf778b2b24\n   3:           0x44cecc - geckodriver::marionette::MarionetteSession::response::h98d048d51a7ee2ba\n   4:           0x42c729 - <webdriver::server::Dispatcher<T, U>>::run::hba9181b5aacf8f04\n   5:           0x402c59 - std::sys_common::backtrace::__rust_begin_short_backtrace::h19de262639927233\n   6:           0x40c065 - std::panicking::try::do_call::h6c1659fc4d01af51\n   7:           0x5e38ec - panic_unwind::__rust_maybe_catch_panic\n                        at /checkout/src/libpanic_unwind/lib.rs:98\n   8:           0x420d32 - <F as alloc::boxed::FnBox<A>>::call_box::h953e5f59694972c5\n   9:           0x5dc00b - alloc::boxed::{{impl}}::call_once<(),()>\n                        at /checkout/src/liballoc/boxed.rs:661\n                         - std::sys_common::thread::start_thread\n                        at /checkout/src/libstd/sys_common/thread.rs:21\n                         - std::sys::imp::thread::{{impl}}::new::thread_start\n                        at /checkout/src/libstd/sys/unix/thread.rs:84"
     }
   }